Coke deposition in heavy oil catalytic hydroprocessing remains a serious problem. The influence of multiphase behavior on coke deposition is an important but unresolved question in this literature. A model mixture comprising Athabasca vacuum bottoms (ABVB) + decane + hydrogen which is shown to exhibit low-density liquid + vapor, high-density liquid + vapor, as well as low-density liquid + high-density liquid + vapor, phase behavior at typical hydroprocessing conditions and a commercial heavy oil hydrotreating catalyst (NiMo/gamma-Al2O3) were employed in this investigation. The influence of multiphase behavior on coke deposition was explored under catalyst coking conditions (380 degrees C and 2 h). The liquid-liquid-vapor region extends from similar to 20% ABVB to similar to 50% ABVB. Coke deposition in the high-density liquid phase was found to be greater than in the low-density liquid phase, at fixed global composition. Conventional kinetics models which do not include the impact of such phase behavior effects cannot account for local maxima in the coke deposition versus coke precursor concentration profiles that result when complex phase behavior is encountered.