Athabasca bitumen was hydrocracked over a commercial NiMo/gamma-Al2O3 catalyst in two reactors, a microbatch reactor and a 1-L continuous stirred tank reactor (CSTR). Coke deposition on catalyst was measured as a function of hydrogen pressure, time on stream, and liquid composition by measuring the carbon content of the cleaned spent catalyst. The carbon content ranged from 11.3% to 17.6% over the pressure range 6.9-15.2 MPa in CSTR experiments. Batch and CSTR experiments showed a rapid approach to a constant coke content with increasing oil/catalyst ratio. Coke deposition was independent of product composition for residue concentrations ranging from 8% to 32% by weight. Removal of the coke by tetralin at reaction conditions suggested reversible adsorption of residue components on the catalyst surface. A physical model based on clearance of coke by hydrogen in the vicinity of metal crystallites is presented for the coke deposition behavior during the first several hours of hydrocracking use. This model gives good agreement with experimental data, including the effect of reaction time, the ratio of total feed weight to catalyst weight, hydrogen pressure, and feed composition, and it agrees with general observations from industrial usage. The model implies that except at the highest coke levels the active surfaces of the metal crystallites remain exposed. Severe mass-transfer limitations are caused by the overall narrowing of the pore structure, which in gamma-Al2O3 would give very low effective diffusivity for residuum molecules in micropores.