The polarographic and voltammetric study of mixed titanium oxide/iron oxide colloids shows that the composition and presumably also the structure of the colloidal particles varies with their size. Iron seems to prevail in the smallest particles; titanium dominates in the surface layers of the bigger ones. Polarographic maxima indicate the particle size. The electroreduction of the trivalent iron in the particles proceeds as a bulk irreversible reaction, while the reduction of the titanium component remains, as in pure TiO2 particles, a partly reversible surface process. In the colloids with low iron content, the reduction of the both electroactive parts occurs independently of each other; however, when the iron content exceeds 50%, the reduction of Fe(III) dominates the mechanism of the Faradaic process.