Hydrogen (H-2) has become a promising energy source because it is clean and has high-energy potential. The aim of this research was to enhance the H-2 production efficiency under anaerobic condition by addition of calcium ions (Ca2+) in a fluidized bed reactor (FBR) with immobilized cells. Ca2+ ions were added either in the form of Ca(OH)(2) or CaCl2. Immobilized cells were prepared by physical adsorption with activated carbon (AC). The experiments were carried out in a FBR system. The H-2 production performance of the FBR fed with sucrose-based synthetic medium, was evaluated under various influent Ca2+ concentrations [Ca2+] (50, 100 and 200 ppm) and hydraulic retention times (HRTs) (8, 6, 4 and 2 h). The peak value of 1.22 L/h-L was obtained at (Ca2+] 100 ppm, irrespective of the form of Ca2+ ion added, and at the HRT of 2 h. Although the results were similar for different forms of Ca2+ ions, the presence of Ca2+ ions enhanced the H-2 producing bioprocess by 12-18%. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.