An improved process is provided for producing porous PVC resin particles by a thickened aqueous suspension polymerization process comprising polymerizing vinyl chloride monomer in an aqueous suspension wherein said aqueous medium contains at least one dispersant capable of thickening water, and optionally at least one secondary surfactant, at least one oil-soluble free radical generating catalyst, the improvement comprising charging said free radical catalyst as a solution wherein the solvent for said catalyst solution has a density equal to or less than that of the monomer being polymerized and the catalyst solution has a density less than water.