Orange- or yellow-orange-colored aqueous dispersions of ultrafine gold particles (similar to 2 nm) result from the gamma-irradiation of deaerated solutions containing hydrolyzed AuCl4- and poly(vinyl alcohol) or poly(vinylpyrrolidone), respectively. The particles have a weak and very broad plasmon absorption band with maximum in the 490 to 500 nm range, i.e., at substantially shorter wavelengths than expected from classical Mie theory using bulk dielectric data; moreover, the specific UV absorption increases as the particles become smaller. The particles are used as seeds in the radiolytic reduction of added Au(CN)(2)(-) to yield larger particles of any desired size and improved monodispersity. The particles partially react with oxygen. Chemisorption of S-mercapto propionic acid strongly affects the electronics of the particles, as indicated by the changes in optical absorption. The early stages of AuCl4- reduction are also investigated. The reduction occurs essentially in two steps: (1) formation, and (2) reduction of Au+. A 203-nm absorption band is tentatively attributed to Au+. Without further irradiation, Au+ disappears thermally within hours to yield larger particles (20-70 nm).