In the presence of semiconductor CdS or ZnO particles, irradiation (>350 nm) of all-trans-beta-carotene (II) in dichloromethane leads to rapid degradation of the carotenoid, which is relatively stable in the absence of the semiconductors. Canthaxanthin (I), however, undergoes significant photocatalyzed degradation only on ZnO, not on CdS. High-performance liquid chromatographic studies indicate that CdS catalyzes trans-cis photoisomerization of both I and II. As in the photoisomerization in the absence of semiconductor, the major cis isomers have the 9-cis and 13-cis configuration, but, under otherwise the same condition, the ratio of cis/trans isomers has doubled. In contrast to CdS, ZnO does not catalyze the photoisomerization of either I or II, although it enhances their rate of degradation. A photoisomerization mechanism involving carotenoid radicals formed by reaction with interstitial sulfur on the CdS surface is proposed.