Photoproduction of H 2 gas has been examined in sulfur/phosphorus-deprived Chalmydomonas reinhardtii cultures, placed in photobioreactors (PhBRs) with different gas phase to liquid phase ratios (V g.p. /V l.p. ). The results demonstrate that an increase in the ratio stimulates H 2 photoproduction activity in both algal suspension cultures and in algae entrapped in thin alginate films. In suspension cultures, a 4× increase (from ∼0.5 to ∼2) in V g.p. /V l.p results in a 2× increase (from 10.8 to 23.1 mmol l −1 or 264–565 ml l −1 ) in the total yield of H 2 gas. Remarkably, 565 ml of H 2 gas per liter of the suspension culture is the highest yield ever reported for a wild-type strain in a time period of less than 190 h. In immobilized algae, where diffusion of H 2 from the medium to the PhBR gas phase is not affected by mixing, the maximum rate and yield of H 2 photoproduction occur in PhBRs with V g.p. /V l.p above 7 or in a PhBR with smaller headspace, if the H 2 is effectively removed from the medium by continuous flushing of the headspace with argon. These experiments in combination with studies of the direct inhibitory effect of high H 2 concentrations in the PhBR headspace on H 2 photoproduction activity in algal cultures clearly show that H 2 photoproduction in algae depends significantly on the partial pressure of H 2 (not O 2 as previously thought) in the PhBR gas phase.