A multistage polymerization protocol, the so-called "modified seeded polymerization," was developed for the production of monodisperse-porous poly(styrene-co-divinylbenzene) providing high column efficiency as a packing material in reversed phase high performance liquid chromatography (RPLC). In the first stage of the multistage production, uniform polystyrene seed particles, produced by dispersion polymerization, were swollen by an organic agent (i.e., the diluent) and then by a monomer mixture containing styrene and divinylbenzene. The final porous particles were obtained in the monodisperse form by the polymerization of monomer mixture in the seed particles. By the use of a small size seed latex with low molecular weight and by the selection of the appropriate diluent, relatively small monodisperse-porous particles with suitable pore structure could be achieved. In the reversed phase separation of alkylbenzenes, under isocratic conditions, theoretical plate numbers up to 40,000 plates/m were achieved by using 5.2 mum porous particles, obtained by a toluene-dibutyl phthalate mixture as the diluent. No significant decrease in the resolution power was observed by the fourfold increase in the mobile phase flow rate. The column efficiency and the resolution observed with 5.2 mum monodisperse-porous particles were significantly higher with respect to the currently available polymer based packing materials used in the reversed phase HPLC. (C) 2005 Wiley Periodicals, Inc.