Ultrafine electrically conductive microparticles have been prepared by in situ oxidative polymerization of 3,4-ethylenedioxythiophene in the presence of sulfonated polystyrene latex beads either in water or in an aqueous solution of poly(4-styrenesulfonic) or 4-toluenesulfonic acid. The particles consist of polystyrene core and continuous poly(3,4-ethylenedioxythiophene) shell which incorporate different counter-ions derived from the oxidant or the sulfonic acid. The presence of poly(4-styrenesulfonic acid) in the polymerization medium enhances the electrical conductivity of the resulting particles by up to two orders of magnitude. The particles prepared in the presence of poly(4-styrenesulfonic acid) exhibit smooth surface morphology and high colloidal stability, while these prepared in the presence of 4-toluenesulfonic acid or in the absence of sulfonic acid show rough sponge-like morphology and do not form stable dispersions.