A generalized methodology has been developed to couple heat and mass transfer of a gas-heavy oil system and a gas mixture-heavy oil system at high pressures and elevated temperatures. Theoretically, the Peng-Robinson equation of state (PR EOS) incorporating with a one-way heat and mass transfer model has been developed to couple heat and mass transfer from either a hot gas or a hot gas mixture into heavy oil. Experimentally, diffusion tests have been conducted with a PVT setup for a hot CO2-heavy oil system and a hot C3H8-CO2-heavy oil system under a constant pressure, respectively. Both the gas-phase volume and liquid-phase swelling effect are simultaneously recorded during the measurement. The gas chromatography method is employed to measure compositions of the C3H8-CO2 mixture at the beginning of the diffusion measurement. The heat transfer is found to proceed faster than mass transfer, leading to that the thermal equilibrium is achieved more quickly than mass equilibrium. The heavy oil expands rapidly at the initial stage of the coupled heat and mass transfer, and then swells gradually in the subsequent stage of mass transfer. The diffusion coefficient is determined by minimizing the discrepancy between the measured and calculated swelling factors of heavy oil during the diffusion tests. Adding C3H8 to CO2 stream is found to not only improve mass diffusion, but also accelerate the heat diffusion and consequently an enhanced swelling effect of heavy oil. (C) 2014 Elsevier Ltd. All rights reserved.