AbstractThe kinetic properties of acetate‐degrading methanogenic granular sludge of different mean diameters were assessed at different up‐flow velocities (Vup). Using this approach, the influence of internal and external mass transfer could be estimated. First, the apparent Monod constant (KS) for each data set was calculated by means of a curve‐fitting procedure. The experimental results revealed that variations in the Vup did not affect the apparent KS‐value, indicating that external mass‐transport resistance normally can be neglected. With regard to the granule size, a clear increase in KS was found at increasing granule diameters. The experimental data were further used to validate a dynamic mathematical biofilm model. The biofilm model was able to describe reaction‐diffusion kinetics in anaerobic granules, using a single value for the effective diffusion coefficient in the granules. This suggests that biogas formation did not influence the diffusion‐rates in the granular biomass. © 2001 John Wiley & Sons, Inc. Biotechnol Bioeng 73: 125–134, 2001.