CdS semiconductor photocatalyst was synthesized by a solvent thermal process. Platinum (Pt) was loaded on the CdS by photodeposition. The samples were characterized by XRD, BET, TEM, DRS and XPS techniques. The activities of Pt/CdS photocatalysts under visible light were evaluated by hydrogen production using formic acid as electron donor. The photoetching of CdS was monitored by measuring dissolved Cd2+. The activity of Pt/CdS with 1 h photoetching treatment increases by 126% compared to the initial photocatalyst. However, excess photoetching decreases the photocatalytic activity. A distinct correlation between photoetching and increase of the activity is observed. Oxygen and platinum play an important role in the photoetching process. The photocatalytic activity is enhanced through decreasing agglomeration of the CdS particles to increase the specific surface area, modifying particle morphology, and removing selectively grain boundary defects which are recombination centers of photoinduced electron-hole pairs. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.