Porous, spherical zirconia particles with a narrow particle size distribution, which are useful as chromatographic packing materials for high performance liquid chromatography (HPLC), were synthesized by polymerization-induced colloid aggregation (PICA) first described by Iler and McQueston (U.S. Patent 4,010,242, 1977.) and the effects of a number of crucial processing variables were examined. In this method, an aqueous zirconia sol consisting of 700 angstrom (mean diameter) particles is mixed with urea and formaldehyde, which are polymerized by the acidic sol. Urea-formaldehyde polymer adsorbs onto the ZrO2 colloids, entraining the colloids in the precipitation of the polymer gel and thus allowing the colloids to aggregate. Features of the aggregation process are elucidated from responses of the process to variations in temperature, reaction mixture composition, and solvent polarity. Our results suggest that the aggregation process resembles those reported for the bridging flocculation of colloids by adsorbed polymers. Porous zirconia particles obtained after polymer combustion and sintering of the aggregates are 3.5 μ;m in diameter with a surface area of 13 m2/g, a porosity of 29% and pores ranging from <50 to 350 angstrom in diameter. The particles are strong enough to withstand the packing of a HPLC column.