A conducting polymer, polypyrrole (PPy), has been deposited from aqueous media onto a submicrometer-sized poly(ethylene glycol)-stabilized polystyrene (PS) latex. Deposition experiments were carried out at two different initial pyrrole concentrations, and the latex concentration was systematically adjusted in order to control the final conducting polymer loading. Transmission electron microscopy studies showed that at a pyrrole concentration of 5.0 x 10(-3) M the PPy had deposited onto the latex as discrete PPy nanoparticles of 20-30 nm diameter. These nanoparticles act as a bridging flocculant or binder, leading to heteroflocculation of the PS latex. Thus the expected "core-shell" morphology was not observed. Pressed pellet conductivity measurements indicated relatively low conductivities (similar to 10(-2) S cm(-1)) for the composites at PPy loadings less than 20 wt %. This is in contrast to earlier studies (Lascelles, S. F.; Armes, S. P. J. Mater. Chem. 1997, 7, 1339) of micrometer-sized PPy-coated PS latexes which exhibited conductivities similar to PPy bulk powder (similar to 1 S cm(-1)) even at PPy loadings as low as 5 wt %. The relative colloid stabilities of the PPy-PS composites were assessed by disk centrifuge photosedimentometry (DCP) studies. For PPy loadings up to 16.6% or lower a reasonable degree of colloid stability was retained, but the PS latex particles were present as stable colloidal microaggregates, which is in good agreement with scanning electron microscopy studies. In contrast, DCP analysis confirmed that a PPy-PS composite prepared using a relatively low latex surface area and a pyrrole concentration of 5 x 10(-4) M comprised mainly singlets, with a much smaller contribution from doublets, triplets, and higher aggregates.