It was demonstrated that immobilized, sulfur-deprived algal cultures can photoproduce H-2. After identifying the optimal material and procedures for immobilization of Chlamyodomonas reinhardtii at high cell density, we examined the effect of liquid mixing, sulfate content, acetate levels and light intensity on the H-2-production activity of the culture. Our results indicate that (a) liquid mixing is important to provide homogeneous conditions for the immobilized culture; (b) sulfur deprivation is necessary for hydrogen production by immobilized cultures; and (c) high light intensity decreases H-2 production. The maximum total volume of H-2 produced by the system (160 ml of reactor volume) was 380 ml over 23 days, and the highest rate of H2 production observed was 45ml day(-1). Cell immobilization significantly increased the duration of the H-2-photoproduction phase (up to 4 weeks), maintained specific rates of H-2 photoproduction similar to those of suspension cultures and showed potential for large increases in H-2 production. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.