In this study, a semi-continuous operation of photo-fermentative H-2-producing reactor was attempted at various decanting volume ratios (DVR, decanting volume per day/total working volume, %), ranging 30-70%, using Rhodobacter sphaeroides KD131. H-2 production was not efficient with showing low H-2 yields of 0.2 and 0.5 mol H-2/mol succinate(added) at 30% and 40% DVR, respectively. The low performance ascribed to the fact that over 70% of substrate electrons were diverted towards cell growth under these conditions. Meanwhile, cell growth was limited at DVR >= 50%; therefore, higher H-2 yields (>2.0 mol H-2/mol succinate(added)) were observed. Both the highest H-2 yield of 3.7 mol H-2/mol succinate(added) and production rate of 1494 mL H-2/L-reactor/d were achieved at 60% DVR. The content of soluble microbial products (SMPs) was measured, which accounted for 3-15% of substrate electrons. It was found that the largest (65-75%) portion of SMPs comprised low molecular-weight (<3 kDa). (C) 2011 Elsevier Ltd. All rights reserved.