This study investigated the influence of H-2 bulk liquid concentration in the range 2-120 mu mol L-1 on the kinetics of vinyl chloride (VC) formation from tetrachloroethene (PCE) and VC dechlorination to ethene in a methanogenic biofilm reactor containing Dehalococcoides spp. as the putative dechlorinating microorganism. Both VC formation and dechlorination showed a definite increase in rate with increasing H-2 bulk liquid concentration, following a pattern typical of Michaelis-Menten kinetics. The estimated maximum VC formation rate (81.7 +/- 9.4 mu mol L-1 h(-1); mean value +/- 90% confidence interval) was about ten times higher than the estimated maximum VC dechlorination rate (8.2 +/- 1.0 mu mol L-1 h(-1)), while the estimated apparent half-velocity coefficient for H-2 for VC formation (1.5 +/- 1.4 mu mo1 H-2 L-1) was more than six times lower than that for VC dechlorination (9.1 +/- 5.1 mu mol H-2 L-1), confirming that the last step of PCE dechlorination (i.e. conversion of VC to ethene) was much more H-2-sensitive than the previous ones. The estimated maximum methane formation rate was 462.1 +/- 213.5 mu mol L-1 h(-1) and the estimated apparent half-velocity coefficient was 104.7 +/- 89.4 gmol H-2 L-1. Experiments at different temperatures indicated the presence of severe internal (diffusional) mass transfer limitations and, in turn, of steep H-2 concentration gradients through the biofilm, which strongly influenced the estimated apparent half-velocity coefficients for H-2 use. The results of this study emphasise the importance of considering mass transfer phenomena when predicting the rate of PCE dechlorination and the outcome of competition for H-2 in natural or engineered bioremediation systems. (c) 2006 Society of Chemical Industry.