Monodisperse 3.2-mum porous poly(styrene-co-divinylbenzene) particles have been prepared by a new multistage polymerization procedure, so-called 'modified seeded polymerization'. In the synthesis of the particles sufficient porosity was generated by use of a diluent mixture with relatively low viscosity. The use of relatively high diluent/seed latex ratio resulted in the formation of particles with an average pore size of 13 nm. The particles were used as column packing material in HPLC and their performance was compared with that of larger particles (d(n) = 7.8-mum) obtained by the same method. The SEC plot for the 3.2 mum particles showed that the proposed material was particularly suitable for separation of analytes in the molecular weight range 100-2000. In reversed-phase mode alkylbenzenes were separated with resolution >2 by use of a relatively small column (50 mm x 4.6 mm i.d.) packed with 3.2- mum particles. Theoretical plate numbers (TPN) in excess of 35 000 plates m(-1) could be achieved with benzene as analyte. Whereas the ratio of TPN determined with pentylbenzene to that determined with benzene was less than 0.7 for the 7.8-mum particles, the same ratio was 0.95 for the 3.2-mum particles. With the 3.2-mum packing material proposed no significant decrease in resolving power was observed when the mobile-phase flow rate was increased sevenfold.