Convective mixing at the edge of the steam chamber enhances heat and mass transfer rates, which increases oil mobility and production rate. A linear stability analysis is performed under transient concentration and steady-state temperature boundary layers with temperature and concentration dependent fluid density and viscosity. It is found that the critical Rayleigh number depends on coefficients of the mixture viscosity and density function. Results show that the onset of instability is delayed as the concentration and temperature dependency of the solvent-oil mixture viscosity increases. Different potential solvents for heavy oil recovery are compared based on the critical time of instability. It is shown that intermediate molecular weight solvents lead to stronger convection, which is in agreement with experimental observations reported in the literature. These results are applicable for optimum design of laboratory experiments and selection of appropriate additives to steam for design of field- scale heavy oil recovery process.