Abstract There is strong evidence in the literature supporting the existence of significant mass transfer limitations on the kinetics of exogenous H 2 consumption by methanogens. The half saturation constant for H 2 uptake by a mixed‐culture, CH 4 producing enrichment was measured using an experimental protocol that avoided internal mass transfer limitations. The value obtained was two orders of magnitude smaller than any other previously reported. A mathematical model for acetogenic syntrophic associations was developed to check the capacity of H 2 as electron transporter between syntrophic partners. It was found that H 2 diffusion could account for the rate of transport of electrons between the syntrophic microorganisms and that formate is not a necessary intermediate. The possibility that formate may be an intermediate in this system was not ruled out. A Monod‐type kinetic equation was modified to include the observed H 2 threshold effect. This modified equation was used to predict the CH 4 ‐production rate in a batch‐fed digester. The results show that the external and internal H 2 pools are kinetically coupled. © 1992 John Wiley & Sons, Inc.