Rough surfaces composed of discrete but relatively uniform nanoparticles were prepared from a lightly sulfortated polystyrene ionomer by spin coating from -tetrahydrofuran (THF) or a THF/methanol mixture onto a silica surface. The particle morphology is consistent with the. spinodal decomposition of the film surface occurring during spin coating. The particles are well wetted to the silica, and if heated for a long time above the ionomer's glass-transition temperature, the particles flow and coalesce into a smooth, homogeneous film.