VAPEX (vapour extraction) is a promising technique for the recovery of heavy oil and bitumen reservoirs, especially for cases where steam-assisted gravity drainage and other thermal recovery methods are not economical. In the VAPEX process, a solvent is injected into the reservoir to reduce the oil viscosity and mobilize it towards the production well. CO2-based VAPEX is an attractive option from both economic and environmental perspectives. In CO2-based VAPEX, unlike other hydrocarbon solvents, dissolution of CO2 into the oil can result in a density increase of the diluted oil. As a consequence, the diluted oil has a higher density than the immobile oil beneath and a gravitationally unstable diffusive boundary layer is induced, which may lead to natural convection. In this study, a mathematical model for the diffusive boundary layer in the CO2-oil contact region in the VAPEX process is developed and the possibility of convective mixing occurrence is examined using linear stability analysis, based on the amplification of the initial velocity perturbations. It is found that in field-scale cases in the VAPEX process, the Rayleigh number is much smaller than the critical Rayleigh number, (Ra-c), and natural convection cannot happen in this process.