Abstract: 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.