A newly isolated Rhodopseudomonas palustris P4 for CO-dependent H-2 production was studied for its capability of fermentative H-2 production in batch cultivations. Important parameters investigated include pH, temperature, concentrations of phosphate and glucose, intermittent purging of culture broth by argon gas, and kind of sugars. The pH of the culture medium significantly decreased as fermentation proceeded due to the accumulation of various organic acids, and this inhibited the H-2 production seriously. The use of fortified phosphate at 60-300 mM could alleviate this inhibition. The increase of glucose concentration (1-20 g/l) resulted in higher H-2 production, but the yield of H-2 production (mmol H-2/mmol glucose) gradually decreased with increasing glucose concentration. Intermittent purging of the culture broth by argon gas improved H-2 production. Carbon mass balance showed that, in addition to cell mass, ethanol, acetate and CO2 were the major fermentation products that comprised more than 70% of carbon consumed. R. palustris P4 could utilize various monosaccharides (glucose, galactose, fructose), disaccharides (lactose, sucrose) and starch to produce H2. However, the H-2 production rate with disaccharides and starch was much slower than that with monosaccharides. The maximal H-2 yield and H2 production rate were estimated to be 2.76 mmol H-2/mmol glucose and 29.9 mmol H-2/g Cell h, respectively. These results indicate that, although isolated for CO-dependent H-2 production, R. palustris P4 has a high potential as a fermentative H-2 producer. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.