Fructose and H-2 were compared as electron donors for hydrogenation of carbon-carbon double bonds using Acetobacterium woodii. Caffeate was used as a model substrate. An electron donor was required and both fructose and H-2 were suitable. With fructose as the donor, the K-s for caffeate was 0.5 mM and the V-max was 678 mmol kg(dry weight)(-1)h(-1). Fructose oxidation was coupled very efficiently to caffeate reduction by an alteration in the fructose fermentation so that acetate was no longer produced.