in preparing microcapsules for carbonless duplicating sheets using aminoaldehyde polycondensate as a capsule wall in a system in which the capsule wall is formed around the hydrophobic material in hydrophilic medium , wherein said hydrophilic medium comprises a combination of an anion - modified polyvinyl alcohol derivative containing sulfo group or sulfonyldioxyd group and an anion - modified water - soluble polymer containing a carboxyl group . the present invention provides a process for producing a high - concentration , low - viscosity microcapsule slurry having advantages such as easy workability , good stability for emulsion , etc .