Disclosed is a process for the preparation of a polymerization process toner for developing an electrostatic image which comprises the polymerization of a polymerizable monomer containing finely divided magnetic grains dispersed therein, a compound represented by formula R--(COOM).sub.n being used as a dispersant, wherein n represents an integer 1 to 4, R represents an alkyl group (including phenyl-substituted alkyl group), an alkylene group (including a phenyl-substituted alkylene group), or a substituted or unsubstituted phenyl group, and M represents a hydrogen atom, alkaline metal or alkaline earth metal. The process provides an enhancement of pigment dispersability in a toner, giving a toner that provides a high image quality.