The co-fermentation of vinasse and cheese whey (CW) was evaluated in this study by using two thermophilic (55 degrees C) anaerobic fluidized bed reactors (AFBRs). In AFBR using vinasse and CW (AFBR-V-CW), the CW was added in increasing proportions (2, 4, 6, 8, and 10 g COD.L-1) to vinasse (10 g COD.L-1) to assess the advantage of adding CW to vinasse. By decreasing the hydraulic retention time (HRT) from 8 h to 1 h in AFBR-V, maximum hydrogen yield (HY), production rate (HPR), and H-2 content (H-2%) of 1.01 +/- 0.06 mmol H-2.g COD-1, 2.54 +/- 0.39 L H-2.d(-1).L-1, and 47.3 +/- 2.9%, respectively, were observed at an HRT of 6 h. The increase in CW concentration to values over 2 g COD.L-1 in AFBR-V-CW decreased the FIY, PVH, and H-2%, with observed maximum values of 0.82 +/- 0.07 mmol H-2.g COD-1, 1.41 +/- 0.24 L H-2.d(-1).L-1, and 55.5 3.7%, respectively, at an HRT of 8 h. The comparison of AFBR-V-CW and AFBR-V showed that the co-fermentation of vinasse with 2 g COD.L-1 of CW increased the HPR, H-2%, and HY by 117%, 68%, and 82%, respectively. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.