Packed-bed bioreactors containing activated carbon as support carrier were used to produce H-2 anaerobically from a sucrose-limiting medium while acclimated sewage sludge was used as the H-2 producer. The effects of bed porosity (epsilon(b)) and substrate loading rate on H-2 fermentation were examined using packed beds with epsilon(b) of 70-90% being operated at hydraulic retention times (HRT) of 0.5-4 h. Higher epsilon(b) and lower HRT favored H-2 production. With 20 g COD l(-1) of sucrose in the feed, the optimal H-2 production rate (7.4l h(-1) l(-1)) was obtained when the bed with epsilon(b)=90% was operated at HRT=0.5 h. Flocculation of cells enhanced the retention of sludge for stable operations of the bioreactor at low HRTs. The gas products resulting from fermentative H-2 production consisted of 30-40% H-2 and 60-70% CO2. Butyric acid was the primary soluble product, followed by propionic acid and valeric acid.