Abstract: The invention relates to a process for preparing vesiculated cross-linked polyester resin granules by polymerizing a water-in-oil-in-water emulsion formed from a polymerizable water-immiscible oil phase, composed of an ethylenically unsaturated monomer and a polyester condensation product and an aqueous phase. The polymerization is effected essentially instantaneously and substantially immediately following the formation of the emulsion by contacting the water-immiscible oil phase at a temperature from 10.degree. to 40.degree.C. with an aqueous phase containing a strong base and having a temperature from 65.degree. to 95.degree.C. Polymerization is thereby effected while the size of the oil phase globules of the water-in-oil emulsion is at a minimum and before appreciable agglomeration can take place, resulting in a reduction in particle size and particle size distribution. The product is especially useful as an opacifying pigment for latex paints and the like, wherein it contributes improved hiding power.