Cheese whey powder (CWP) solution was used as the raw material for hydrogen gas production by mesophilic (35 degrees C) and thermophilic (55 degrees C) dark fermentations at constant initial total sugar and bacteria concentrations. Thermophilic fermentation yielded higher cumulative hydrogen formation (CHF = 171 mL), higher hydrogen yield (111 mL H-2 g(-1) total sugar), and higher hydrogen formation rate (3.46 mL H-2 L-1 h(-1)) as compared to mesophilic fermentation. CHF in both cases were correlated with the Gompertz equation and the constants were determined. Despite the longer lag phase, thermophilic fermentation yielded higher specific H-2 formation rate (2.10 mL H-2 g(-1) cells h(-1)). Favorable results obtained in thermophilic fermentation were probably due to elimination of H-2 consuming bacteria at high temperatures and selection of fast hydrogen gas producers. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.