The photocorrosion behavior of CdS nanoparticle colloid containing particles ranging from 4 to 10 nm was investigated using two kinds of monochromatic light, one of which was prepared by passing light from a Xe lamp through interference filters, and the other was obtained by an Ar ion laser. CdS particles were photocorroded to smaller particles until irradiated photons were not absorbed anymore; and the CdS particles obtained after the complete photocorrosion by laser light were much smaller than those resulting from the use of monochromatic light of comparable wavelength. The size selective photocorrosion with the Ar ion laser allowed the preparation of much smaller particles than those in the original colloid. It was found that the rate of photocorrosion decreased when the pH of the colloid was raised, while the fluorescence intensity due to bandgap transition increased. The importance of hole trap sites in the photodissolution of CdS is discussed based on these findings.