Melis et al. have demonstrated that the green alga Chlamydomonas reinhardtii, when deprived of sulfur, can produce hydrogen gas for similar to70 h, then can resume hydrogen gas production after a brief period of "recharging" in the presence of sulfur. Here we describe an S-system model of H-2 production by C reinhardtii. Through that model we investigate the sensitivity of H-2 production to photosynthetic efficiency, and to contention for the protons produced by the photolysis of water, between hydrogen production on the one hand, and ATP consumption by cellular functions outside the H-2 production path on the other. The model identifies for experimental investigation several potential systemic constraints on any genetic re-engineering effort aimed at increasing the H-2 production efficiency of the alga. (C) 2002 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy.