In this paper, the so-called pressure decay method is applied to measure the molecular diffusivities of carbon dioxide, methane, and propane in heavy oil. In the experiment, a gaseous solvent is made in contact with a heavy oil, and thereby, the pressure in the solvent phase versus time data are accurately measured inside a closed high-pressure diffusion cell at a constant temperature while the solvent gradually dissolves into the heavy oil. In terms of the conservation law of mass and the equation of state for a real gas, the pressure in the solvent phase is calculated from the analytical solution to the diffusion equation for such a diffusion process. The equilibrium, quasi-equilibrium, and nonequilibrium boundary conditions are applied at the heavy oil-solvent interface, respectively. The solvent diffusivity in heavy oil is determined by finding the best match of the numerically calculated pressures with the experimentally measured data. It is found that the nonequilibrium boundary condition is the most applicable at the heavy oil-CO2 interface at small diffusion times. In addition, the determined diffusivity for the heavy oil-CH4 system is insensitive to the interface boundary condition. The mass transfer across the heavy oil-C3H8 interface is best described by applying the quasi-equilibrium boundary condition. In particular, a new strategy is adopted to find the equilibrium pressure for each heavy oil- solvent system from its measured solubility versus pressure data. Thus, the diffusion coefficient of each solvent in heavy oil can be determined by measuring the pressure decay in a short duration.