Two characteristic temperatures were identified from measurements of the temperature dependence of 02 evolution by Chlorella vulgaris and Anacystis nidulans: T1, the threshold temperature for inhibition of 02 evolution under saturating light conditions, and T2, the upper temperature limit for 02 evolution. Measurement of delayed light emission from photosystem II (PSII) showed that it passed through a maximum at T~ and was virtually eliminated on heating the samples to T2. Related changes were observed in low-temperature (77K) fluoresence emission spectra. Heatstress had little effect on the absorption properties of the cells at temperatures below T~ but incubation at higher temperatures, particularly under high-light conditions, resulted in extensive absorption losses. An analysis of these measurements suggests that this increased susceptibility to photobleaching is triggered by an inhibition of the flow of reducing equivalents from PSII that normally serves to protect the light-harvesting apparatus of the cells from photo-oxidation. Adaptation to higher growth temperatures resulted in increases in the values of T~ and T 2 for Anacystis nidulans but not for Chlorella vulgaris.