Fixing carbon dioxide (CO2) with solar hydrogen (H-2) is a novel alternative to conventional photosynthesis of plants and microalgae. The energy efficiency of CO2 fixation by a hydrogen-oxidizing bacterium was investigated in a closed reactor system. The molar ratio of consumed H-2 and CO2 was measured under mass transfer limitation in atmospheres of sufficient H-2, low CO2, and a broad range of O-2. The energy efficiency, ranging from 10% to 60%, was primarily affected by the oxygen concentration (6-30 mol%). The research revealed a clear trend that a low oxygen concentration gave high energy efficiency, but slow gas consumption. A high energy efficiency of 50% was measured under a moderate oxygen concentration (10 mol%). Based on 10% solar hydrogen efficiency, a 5% overall efficiency from solar energy to biomass can therefore be achieved. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.