The adsorption of cellulase from Trichoderma reesei MCG 77 on Avicel was measured at varying cellulase (2–8 g/L) and Avicel (10–200 g/L) concentrations at pH 4.8 and 50°C. Different mathematical equations were derived for the evaluation of the experimental data. The fraction of cellulase protein that can maximally be adsorbed is 0.96, and 1 g Avicel can bind maximally 0.092 g cellulase protein. The Michaelis constant for the adsorption equilibrium [cellulase] + [Avicel] ⇌ [cellulase Avicel] complex is between 2.0 and 2.3 · 10−5 mol/L. This value is based on the assumption that cellulase has an average molecular weight of 48.000. The apparent molecular weight of Avicel, i.e., that amount in grams that can bind 1 mol cellulase, is 520,000. Under maximum binding the enzyme covers on Avicel a surface of 42 m2/g, and the occupied volume is 0.186 cm3/g Avicel.