Narrowing the size distribution of polydisperse CdS nanocrystals whose average diameter was 42 Angstrom and standard deviation was 19 Angstrom was achieved utilizing size selective photocorrosion with sequential irradiation with monochromatic light whose wavelength was changed step by step from 490 to 430 nm in air-saturated sodium hexametaphosphate solution. With decreasing the wavelength of irradiated light, the first exciton peak was gradually developed in the absorption spectrum of the resulting CdS colloid, and the nearly monodisperse CdS nanocrystals of 22 Angstrom were finally obtained, which were thought to be the smallest particles that were present in the original CdS colloid. Analyses of the amount of sulfate ions produced by photocorrosion of Q-CdS colloids revealed that the number of Q-CdS particles in the colloid decreased with promotion of photocorrosion, suggesting that during the course of photocorrosion photocorroded CdS particles were agglomerated to give larger particles which were further photocorroded. The molar absorption coefficient of CdS particles at the first exciton peak was found to be independent of the particle size.