The objective of this study was to investigate the effects of hydraulic retention time (HRT) (8 to 0.5 h) and temperature (55 to 75 degrees C) in two anaerobic fluidized bed reactors (AFBR) using cheese whey (AFBR-CW = 10,000 mg sugars L-1) and vinasse (AFBR-V = 10,000 mg COD L-1) as substrates. Decreasing the HRT to 0.5 h increased the hydrogen production rates in both reactors, with maximum values of 5.36 +/- 0.81 L H-2 h(-1) L-1 in AFBR-CW and 0.71 +/- 0.16 L H-2 h(-1) L-1 in AFBR-V. The optimal conditions for hydrogen production were the HRT of 4 h and temperature of 65 degrees C in AFBR-CW, observing maximum hydrogen yield (HY) of 5.51 +/- 0.37 mmol H-2 g COD-1. Still, the maximum HY in AFBR-V was 1.64 +/- 0.22 mmol H-2 g COD-1 at 4 h and 55 degrees C. However, increasing the temperature to 75 A degrees C reduced the hydrogen production in both reactors. Methanol and butyric, acetic, and lactic acids were the main metabolites at temperatures of 55 and 65 degrees C, favoring the butyric and acetic metabolic pathways of hydrogen production. The increased productions of lactate, propionate, and methanol at 75 degrees C indicate that the hydrogen-producing bacteria in the thermophilic inoculum were inhibited under hyperthermophilic conditions.