Keulemans regularly provided illustrations for The Ibis and The Proceedings of the Zoological Society.
He illustrated many important bird books, including Buller's A History of the Birds of New Zealand (1873, 1888), William Vincent Legge's History of the Birds of Ceylon (1880), Daniel Giraud Elliot's Monograph of the Bucerotidae (hornbills) (1887–1892), Henry Seebohm's Monograph of the Turdidae (thrushes) (1902), Osbert Salvin's Biologia Centrali-Americana (1879–1904), Edgar Leopold Layard's Birds of South Africa (1887) and Henry Eeles Dresser's History of the Birds of Europe (1871–1896), and a single illustration in The Journal of the Linnean Society.

One of his last great achievements was his contribution of over one hundred plates for Frederick Du Cane Godman's Monograph of the Petrels (1907–1910).
He also spent some time collecting birds in Cape Verde and West Africa.

Keulemans is credited with describing the Cape Verde swamp-warbler, Calamodyta (Acrocephalus) brevipennis.
 This is a drab bird about 14–16 cm.,
light brown above and on its flanks, and buff below.
 He did not publish an illustration of it, but his plate for Acrocephalus brunnescens in George Henderson's Lahore to Yarkand (pl.
XVI) is similar.

His notes and findings on the island of Principe, along with those of his colleague Dr. H. Dohrn, would eventually become the basis for a later description of a rare ibis, Lampribis rothschildi Bannerman.

The only significant biography of Keulemans is by Jan Coldewey and Tony Keulemans, Feathers to Brush, a book that includes a bibliography of the artist's publications, a genealogical tree and appendices detailing his spiritualism, with a sample of his financial correspondence.
Also of note is a contemporary obituary of Keulemans in the journal British Birds (1912).
 Tony Keulemans later wrote Beyond the grave, which tells the story of a remarkable discovery of a painting John Gerrard had made of his own gravestone.
 And finally, Tony Keulemans wrote an errata list to Feathers to Brush, which includes additional literature references and new genealogical findings.

Keulemans's work is characterised by its consistency, showing little change over the course of his career, and focused to an extraordinary degree on the rendering of fine detail.
 These generalisations have also proven to be the basis for unjustified criticism of his work, since the nature of scientific illustration places a premium on consistency.
 Aside from this, a number of critics have rightly placed Keulemans above his contemporaries; his ability to create accurate and vivid representations of birds gave him prominence in his field.

Keulemans was prodigious in his output - he was commissioned to paint pictures of birds extensively throughout his career, and his prints were published continuously from 1867 to 1911.
 Keulemans' first prints appeared in two books by Francois Pollen, Contributions a l'histoire naturelle des Lemuriens (1867) and Een blik in Madagascar (1867).
 Some appeared after his death until 1915 (Mathews, Birds of Australia); he had rendered the images on stone well before publication of these works.
 A calculation of his total output gives about 4,000-5,000 published illustrations.
 The vast majority of these were vignettes published within octavo-size books and publications, and a great number of his works also appeared in quarto (Dresser/Europe) and in folio (Seebohm/Turdidae and DuCane Godman/Petrels).
 While the subject of his illustrations was almost entirely avian, he was also commissioned to create portraits of mammals, insects, and shells.

Most of the illustrations by Keulemans were produced through traditional lithography, allowing for a finished product that depicts a vivid, lifelike figure through depth and tone.
Printing was carried out by the two firms of Mintern and Hanhart, and early in his career, some were printed by P. M. W. Trap.
 Often, the published lithographs were not coloured, and perhaps some (Journal of the Linnean Society, 1878) were not intended to be coloured.
The technique of lithography made it neces
Keulemans regularly provided illustrations for The Ibis and The Proceedings of the Zoological Society. He illustrated many important bird books, including Buller's A History of the Birds of New Zealand (1873, 1888), William Vincent Legge's History of the Birds of Ceylon (1880), Daniel Giraud Elliot's Monograph of the Bucerotidae (hornbills) (1887–1892), Henry Seebohm's Monograph of the Turdidae (thrushes) (1902), Osbert Salvin's Biologia Centrali-Americana (1879–1904), Edgar Leopold Layard's Birds of South Africa (1887) and Henry Eeles Dresser's History of the Birds of Europe (1871–1896), and a single illustration in The Journal of the Linnean Society.
One of his last great achievements was his contribution of over one hundred plates for Frederick Du Cane Godman's Monograph of the Petrels (1907–1910). He also spent some time collecting birds in Cape Verde and West Africa.
Keulemans is credited with describing the Cape Verde swamp-warbler, Calamodyta (Acrocephalus) brevipennis.  This is a drab bird about 14–16 cm., light brown above and on its flanks, and buff below.  He did not publish an illustration of it, but his plate for Acrocephalus brunnescens in George Henderson's Lahore to Yarkand (pl. XVI) is similar.
His notes and findings on the island of Principe, along with those of his colleague Dr. H. Dohrn, would eventually become the basis for a later description of a rare ibis, Lampribis rothschildi Bannerman.
The only significant biography of Keulemans is by Jan Coldewey and Tony Keulemans, Feathers to Brush, a book that includes a bibliography of the artist's publications, a genealogical tree and appendices detailing his spiritualism, with a sample of his financial correspondence. Also of note is a contemporary obituary of Keulemans in the journal British Birds (1912).  Tony Keulemans later wrote Beyond the grave, which tells the story of a remarkable discovery of a painting John Gerrard had made of his own gravestone.  And finally, Tony Keulemans wrote an errata list to Feathers to Brush, which includes additional literature references and new genealogical findings.
Keulemans's work is characterised by its consistency, showing little change over the course of his career, and focused to an extraordinary degree on the rendering of fine detail.  These generalisations have also proven to be the basis for unjustified criticism of his work, since the nature of scientific illustration places a premium on consistency.  Aside from this, a number of critics have rightly placed Keulemans above his contemporaries; his ability to create accurate and vivid representations of birds gave him prominence in his field.
Keulemans was prodigious in his output - he was commissioned to paint pictures of birds extensively throughout his career, and his prints were published continuously from 1867 to 1911.  Keulemans' first prints appeared in two books by Francois Pollen, Contributions a l'histoire naturelle des Lemuriens (1867) and Een blik in Madagascar (1867).  Some appeared after his death until 1915 (Mathews, Birds of Australia); he had rendered the images on stone well before publication of these works.  A calculation of his total output gives about 4,000-5,000 published illustrations.  The vast majority of these were vignettes published within octavo-size books and publications, and a great number of his works also appeared in quarto (Dresser/Europe) and in folio (Seebohm/Turdidae and DuCane Godman/Petrels).  While the subject of his illustrations was almost entirely avian, he was also commissioned to create portraits of mammals, insects, and shells.
Most of the illustrations by Keulemans were produced through traditional lithography, allowing for a finished product that depicts a vivid, lifelike figure through depth and tone. Printing was carried out by the two firms of Mintern and Hanhart, and early in his career, some were printed by P. M. W. Trap.  Often, the published lithographs were not coloured, and perhaps some (Journal of the Linnean Society, 1878) were not intended to be coloured. The technique of lithography made it neces
John Gerrard Keulemans