As one of the hybrid steam-solvent processes, hot solvent injection is a promising heavy oil recovery technique. It takes advantage of both thermal recovery processes (quick thermal conduction and viscosity reduction) and solvent-based processes (lower energy consumption and less green-house gas emission). In comparison with conventional cold solvent processes, hot solvent processes can greatly improve oil production. This paper develops a transient mass transfer model to analyze the heavy oil-hot solvent mixing process during a hot solvent injection process. Modeling results show that a moderate elevation in temperature leads to a slight improvement in solvent dissolution but a large enhancement in oil drainage. It is found that the gravity drainage rate of solvent-diluted heavy oil depends strongly on the heated oil viscosity. Furthermore, it shows that temperature falloff slows down oil drainage, which suggests that temperature should be maintained stable during a hot solvent injection process.