the instant invention is directed to a molecular sieve comprising a carbonaceous substrate selected from the group consisting of molecular sieve carbon , calcined coconut char , oxidized and devolatilized agglomerated coal , calcined agglomerated coal and calcined agglomerated coconut char , impregnated with an organic polymer having a molecular weight of at least 400 , as determined by vapor phase osmometry , or with an inorganic polymer at a dosage of at least 0 . 001 weight percent up to a maximum dosage which still allows the resulting sieve to separate gas or liquid mixtures containing components of at least two different molecular diameters , molecular weights or molecular shapes . the instant invention is also directed to a process for preparing the molecular sieve and to a process for using it to separate gas or liquid mixtures .