We report the optical properties of nanosized (approximate to 4 nm) platinum particles synthesized in water-in-oil (w/o) microemulsions. The particles present a maximum at 220-256 nm, being highly position dependent upon the reaction medium. The aging of solutions causes cluster formation, whose absorption spectrum is rather different from those of the initial solutions with a second smaller maximum at longer wavelengths. In the extreme case when particles are separated from microemulsions and redispersed in water, a flat spectrum with a maximum at 260 nm appears. Changes in the optical absorption during reaction are used to propose a formation and stabilization mechanism of the particles. A comparison with the results of platinum particles obtained in water is also reported. Colloidal platinum was also obtained without hydrazine by heating. The optical properties of the metallic particles are identical with the properties of the corresponding particles prepared with hydrazine. This reaction is catalyzed by light. Various surfactants were used in order to propose a mechanism for the formation of the colloidal particles by this reaction pathway.