Small colloidal particles of a semiconductor nature yield polarographic and voltammetric diffusion-controlled currents which differ, by the form as well as by the information contents, from the currents obtained with true solutions. The electrode reaction proceeds here either as a volume or as a surface process according to whether Faradaic charge is transferred to the conduction band and consumed in the volume of the particle or is transferred to and consumed in the surface of the particle. In polydisperse colloidal solutions each particle contributes to the net current according to its size; different size implies different diffusion coefficient, different Faradaic charge, and different reduction/oxidation potential. Hence, in polarography the slope of the wave and the half-wave potential and in voltammetry the peak potential depend on the particle size distribution. Basic experimental material was gathered with ultrasmall SnO2, TiO2, and mixed TiO2Fe2O3 colloids. In the electroreduction of protonated SnO2 and TiO2 colloids in acidic solutions, where the electrode process starts by reduction of the surface protons, the smaller particles are reduced at less negative potentials than the bigger ones.