The protection of chlorophyll a from red light (λ > 630 nm) induced photobleaching was compared for 9-cis- and all-trans-sz-carotene in 2-propanol, acetone, toluene and n-octanol. The composition of the reaction mixtures was followed by absorption spectroscopy and high performance liquid chromatography. Photoprotection was negligible in n-octanol. In the other three solvents, the photoprotection of chlorophyll a with all-trans-sz-carotene increased with decreasing viscosity and up to saturation with increasing concentration of the carotene. However, both pigments were degraded with similar kinetics. Product analysis showed that the photoprotection was limited by oxidation of all-trans-sz-carotene to a product which itself acted as a oxidant to chlorophyll a. Photoprotection by 9-cis-sz-carotene was in acetone 50% more efficient than that by the all-trans-isomer, but became limited during irradiation due to isomerization of the 9-cis to the all-trans isomer. The data show that under appropriate conditions 9-cis-sz-carotene can be a better photoprotectant than the all-trans isomer.