Up to now, the analysis of the effects of medium composition on biohydrogen production of Caldicellulosiruptor saccharolyticus was focused mainly on salt concentrations and complex compounds. Within this work we studied the effects of the presence of organic and/or inorganic nitrogen in the medium composition aiming to induce metabolic changes in C. saccharolyticus to improve its hydrogen evolution rate (HER) and hydrogen specific productivity (q(H2)). Biohydrogen productivities and hydrogen to substrate yield (Y-(H2/5)) of C. saccharolyticus on xylose in batch mode were higher working in a complex medium than in a defined one; but no significant difference could be settled according to hydrogen to carbon dioxide yields (Y-(H2/CO2)). The specific growth rate of C. saccharolyticus on complex medium was settled at 0.1 h(-1) operating in chemostat mode to achieve the highest H-2-productivities under stable conditions. In chemostat mode on xylose, a reduction of the ammonium feed concentration in a defined medium until N-limiting conditions involved higher q(H2) comparing with a straight C-limiting growth. In this contribution we present a methodology for quantitative process characterization and development based on physiological parameters, such as specific rates and yields. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.