Simultaneous photoevolution of hydrogen and oxygen by an aqueous suspension of the unicellular green alga Chlamydomonas reinhardii was measured in a confined bioreactor. The headspace above the liquid was about three times the volume of the liquid. The objective of the experiments was to determine the extent to which Henry's Law partitioning between the liquid and gas phases of the reactor volume can be used to minimize the antagonistic effect of photosynthetically produced oxygen on the evolution of molecular hydrogen by photosynthesis. The results indicate, to within upper limits, that light-driven oxygen and hydrogen produced by microalgal water oxidation in the aqueous phase can partition favorably into the gas phase such that photosynthetically produced oxygen does not inhibit the hydrogenase enzyme. Continual cycles of light-driven hydrogen and oxygen production, with intervals of added gas-phase carbon dioxide for rejuvenating the algae, were performed for about 60 days on the same culture. Absolute yields of hydrogen and oxygen plus computed stoichiometric ratios are reported. The average stoichiometric ratio of hydrogen to oxygen was 2.8, indicating that reducing equivalents for molecular hydrogen were derived from endogenous reductants, most likely starch, as well as water. Additional experiments on the effect of illumination and dwell time on hydrogen and oxygen yields are presented, as are "before" and "after" photomicrographs of the algae, illustrating the effects of prolonged anaerobiosis on the cells. A mathematical model of gas formation in the liquid phase and its equilibrium with the headspace above the liquid is presented. An estimate of the mass transfer coefficient on movement of oxygen from the liquid phase into the gas phase yielded a time constant of less than 2 s. This characteristic time is much shorter than the time scale over which the accumulated yields of hydrogen and oxygen are measured. The data suggest that Henry's Law partitioning may be a rational approach to addressing the issue of oxygen sensitivity of algal hydrogen production, especially when combined with C. reinhardtii mutants that are selected for improved oxygen tolerance.