Interest in renewable and clean energies such as hydrogen has increased because of the high level of polluting emissions, increasing costs associated with petroleum and the escalating problems of global climate change. In the presence of a light source, a microbial photosynthetic process provides a system for the conversion of some organic compounds into biomass and hydrogen. Using Rhodopseudomonas palustris as a cell-factory, hydrogen photo-evolution was investigated in a photobioreactor (PBR) irradiated either from one or two opposite sides. Irradiating the photobioreactor from only one side, in the presence of malic acid, a reactor hydrogen production of 2.786 I(H-2) PBR-1 was achieved. When the PBR was irradiated from two opposite sides, hydrogen photo-evolution increased to 3.162 I(H-2) PBR-1. Experiments were carried out using inoculum from either the retardation or the exponential growth phases. Using the latter, the highest hydrogen photo-evolution rate based on the bacteriochlorophyll (Bchl) concentration was achieved (3295 mu l(H-2) mg (Bchl(-1) h(-1)). The hydrogen to biomass ratio (r(g)) was 1.91 l g(-1) in the medium containing malic acid and 1.07 l g(-1) in that containing acetic acid. It was found that the hydrogen production rate was higher with malic than with acetic acid. Although photobiological hydrogen production cannot furnish alone the greater and greater world requirements of clean renewable energy, it is desirable that photobiological hydrogen technology will grow, in the near future, because photobioreactors for bio-hydrogen production can be positioned in fringe areas without competition with agricultural lands. (C) 2009 Elsevier Ltd. All rights reserved.