Photoreduction of methyl viologen (MV(2+)) and propyl viologen sulfonate (PVS0) on quantized CdS particles (Q-CdS), fractionated using gel electrophoresis, was investigated by irradiation with the third harmonic of a Q-switched Nd:YAG laser. The rate of reduction of MV(2+) increased with decreasing particle size, while that of PVS0 having a redox potential close to that of MV(2+) was independent of particle size. Fluorescence quenching of Q-CdS followed a Stern-Volmer relation when PVS0 was used as a quencher and static quenching when MV(2+) was used. The potential of the conduction band edge of the size fractionated nanocrystals relative to that of bulk CdS was estimated on the basis of the flat band potential data of bulk CdS and the shifts of the conduction band expected by the size-quantization. Assuming that potential differences between the potential of the conduction band edge and the redox potential of MV(2+) give the overvoltage for the reduction reaction, a Tafel relation having the transfer coefficient of ca. 0.3 is derived for photoreduction of MV(2+). The appearance of the Tafel relation under high overvoltages is rationalized for the photoreduction of adsorbed MV(2+) with pulsed irradiation.