The kinetic properties of acetate-degrading methanogenic granular sludge of different mean diameters were assessed at different up-flow Velocities (V-up). Using this approach, the influence of internal and external mass transfer could be estimated. First, the apparent Monod constant (K-S) for each data set was calculated by means of a curve-fitting procedure. The experimental results revealed that variations in the V-up did not affect the apparent K-S-value, indicating that external mass-transport resistance normally can be neglected. With regard to the granule size, a clear increase in K-S 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.