We have studied the effect of viscosity on natural convection in the boundary layer of the vapor extraction (VAPEX) process. VAPEX is a heavy oil recovery method that uses solvents to reduce oil viscosity, and is a potential process in reservoirs where thermal recovery methods cannot be applied. Natural convection may happen in VAPEX if the solvents that are used to decrease oil viscosity increase the density of the oil. This can especially occur with -based solvents. Reduction of the oil viscosity due to solvent dissolution can have a large impact on the onset of convection by decreasing the critical Rayleigh number. When the viscosity reduction is significant, the critical Rayleigh can decrease up to two orders of magnitude. The transverse Peclet number is also a crucial parameter in determining the critical Rayleigh and onset of convection. Our analysis shows that the longitudinal Peclet does not have a significant impact on the natural convention in VAPEX. When oil viscosity reduction is included in the analysis of boundary layer instability in VAPEX, natural convection may occur in high-permeable reservoirs (where Rayleigh number is high) leading to a greater oil production rate compared with current models where the effect of boundary layer instability has been ignored.