To evaluate the influence of hydraulic retention time (HRT) and cheese whey (CW) substrate concentration (15 and 25 g lactose l(-1)) on the performance of EGSB reactors (R15 and R25, respectively) for H-2 production. A decrease in the HRT from 8 to 4 h favored the H-2 yield and H-2 production rate (HPR) in R15, with maximum values of 0.86 +/- 0.11 mmol H-2 g COD-1 and 0.23 +/- 0.024 l H-2 h(-1) l(-1), respectively. H-2 production in R25 was also favored at a HRT of 4 h, with maximum yield and HPR values of 0.64 +/- 0.023 mmol H-2 g COD-1 and 0.31 +/- 0.032 l H-2 h(-1) l(-1), respectively. The main metabolites produced were butyric, acetic and lactic acids. The EGSB reactor was evaluated as a viable acidogenic step in the two-stage anaerobic treatment of CW for the increase of COD removal efficiency and biomethane production.