Abstract: The present invention relates to a process for preparing polymeric particles which is specially designed to be served as an opacifying agent in coating compositions such as water-borne paints, paper coating compositions and molding compositions which have white or light color shades. In more detailed description, this invention is concerned with the process for making an aqueous dispersion of core-shell emulsion polymer particles comprising (1) preparing the core by polymerizing carboxylic acid monomers with other monoethylenically unsaturated monomers, (2) encapsulating said core with a rigid polymer shell by polymerizing shell-forming monomers, in the presence of said core particles, which are fed in such manner that compositional contents of monomers entering to a polymerization zone is changing gradually thoughout the period of monomer feeding, and (3) neutralizing with base so as to swell said core and form particles which, when dried, contain at least one void.