The recently developed two–stage photosynthesis and H 2 –production protocol with green algae is further investigated in this work. The method employs S deprivation as a tool for the metabolic regulation of photosynthesis. In the presence of S, green algae perform normal photosynthesis, carbohydrate accumulation and oxygen production. In the absence of S, normal photosynthesis stops and the algae slip into the H 2 –production mode. For the first time, to our knowledge, significant amounts of H 2 gas were generated, essentially from sunlight and water. Rates of H 2 production could be sustained continuously for ca . 80 h in the light, but gradually declined thereafter. This work examines biochemical and physiological aspects of this process in the absence or presence of limiting amounts of S nutrients. Moreover, the effects of salinity and of uncouplers of phosphorylation are investigated. It is shown that limiting levels of S can sustain intermediate levels of oxygenic photosynthesis, in essence raising the prospect of a calibration of the rate of photosynthesis by the S content in the growth medium of the algae. It is concluded that careful titration of the supply of S nutrients in the green alga medium might permit the development of a continuous H 2 –production process.