Syntrophospora bryantii degraded butyrate in co-culture with methanogens that can use both H-2 and formate for growth, but not in co-culture with methanogens that metabolize only H-2, suggesting that in suspended cultures formate may be a more important electron carrier in the syntrophic degradation of butyrate than H-2. Syntrophic butyrate oxidation was inhibited by the addition of 20 mM formate or the presence of 130 kPa H-2. In the absence of methanogens, S. bryantii is able to couple the oxidation of butyrate to acetate with the reduction of pentenoate to valerate. Under these conditions, up to 300 Pa H-2 was measured in the gas phase and up to 0.3 mM formate in the liquid phase. S. bryantii was unable to grow syntrophically with the aceticlastic methanogen Methanothrix soehngenii. However in triculture with Methanospirillum hungatei and Methanothrix soehngenii, S. bryantii degraded butyrate faster than in a biculture with only M. hungatei. Hydrogenase and formate dehydrogenase activities were demonstrated in cell-free extracts of S. bryantii.