This study focuses on the exploitation of cheese whey as a source for hydrogen and methane, in a two-stage continuous process. Mesophilic fermentative hydrogen production from undiluted cheese whey was investigated at a hydraulic retention time (HRT) of 24 h. Alkalinity addition (NaHCO(3)) or an automatic pH controller were used, to maintain the pH culture at a constant value of 5.2. The hydrogen production rate was 2.9+/-0.2 L/Lreactor/d, while the yield of hydrogen produced was approximately 0.78+/-0.05 mol H(2)/mol glucose consumed, with alkalinity addition, while the respective values when using pH control were 1.9+/-0.1 L/Lreactor/d and 0.61+/-0.04 mol H(2)/mol glucose consumed. The corresponding yields of hydrogen produced were 2.9 L of H(2)/L cheese whey and 1.9 L of H(2)/L cheese whey, respectively. The effluent from the hydrogenogenic reactor was further digested to biogas in a continuous mesophilic anaerobic bioreactor. The anaerobic digester was operated at an HRT of 20 d and produced approximately 1L CH(4)/d, corresponding to a yield of 6.7 L CH(4)/L of influent. The chemical oxygen demand (COD) elimination reached 95.3% demonstrating that cheese whey could be efficiently used for hydrogen and methane production, in a two-stage process.