The aim of this study was to determine the competition between H-2 production and polyhydroxybutyrate (PHB) accumulation of Rhodopseudomonas palustris WP3-5 when grown on six different substrates. From the results, strain WP3-5 can utilize acetate, propionate, malate, and lactate to produce H-2 but can only synthesize PHB on acetate and propionate. The substrate conversion efficiency (SCE) on acetate and propionate increased significantly after the maximum PHB content was achieved, illustrating a competition for reducing power when PHB synthesis occurred. However, when strain WP3-5 was cultivated at suboptimal pH values on acetate, the synthesized PHB prevented strain WP3-5 from the stress of the inappropriate pH and retained H-2 producing efficiency as at optimal pH value. Consequently, although PHB synthesis does compete with H-2 production in R. palustris WP3-5, it is still conducive to H-2 production when strain WP3-5 is in a stressful condition. (C) 2012 Elsevier Ltd. All rights reserved.