The particle formation of azobenzene-containing ionomers, through self-assembly, in aqueous media (THF/H2O solvent) was studied. The ionomers were synthesized by copolymerization between azobenzene-pendent methacrylate and methacrylic acid. It was revealed by UV-vis spectra and light scattering measurements that the extent of H-aggregation of the azobenzene units first decreased, and then increased with increasing volume fraction of H2O of the solvent. The H2O fraction at which the extent of H-aggregation began to increase became lower, when the copolymers contained more azobenzene unite. Colloidal particles were prepared by slow addition of various concentrations of aqueous NaOH to the copolymer THF solutions. The hydrodynamic diameters of the particles obtained by the procedures were several hundreds of nanometer. When the azobenzene unit content in the copolymer was smaller, the diameters of the particles became smaller. When the colloidal particles dispersions were cast on a carbon sheet and dried, the particles aggregated and formed larger spherical particles, with diameters of several micrometers. The size of the particles obtained by the drying process became smaller, when higher concentrations of aqueous NaOH solutions were used. Therefore, the particle sizes were controlled by the azobenzene units content in the copolymers and the concentration of aqueous NaOH solutions. (c) 2006 Wiley Periodicals, Inc.