The goal of the study was to show that immobilized purple bacteria could photoproduce H-2 using dark fermentation effluent (FE) as substrate. Simple pretreatment of an inexpensive glass-fiber matrix accelerated the immobilization process. Photobioreactors (PhBR) containing immobilized Rhodobacter sphaeroides GL produced 0.128 L H-2 h(-1) L-1 of PhBR volume (0.570 L h(-1) L-1 of matrix) for up to 3 months when continuously fed artificial media with volatile fatty acids (VFAs) or FE from potato and starch fermentations. Hydrogen production was insensitive to NH4+ up to 1 mM and saturated at 8 mM lactate or 1.5% potato FE (diluted in water and supplemented with critical micronutrients). The efficiency of VFA transformation to H-2 was 50-70% of theoretical. At nonlimiting substrate concentrations in artificial media or FE, acetate was utilized before butyrate. High volumetric rates of continuous H-2 photoproduction and stability of the process are advantages of using immobilized cultures. Use of H-2 photoproduction as a polishing step in the treatment of FEs from dark fermentations increased the total amount of H-2 produced from 0.9 to 4.7 mol mol(-1) glucose equivalent in the original potato homogenate. (C) 2011 American Institute of Chemical Engineers Biotechnol. Prog., 27: 1248-1256, 2011