Abstract Expanding-Solvent Steam-Assisted Gravity Drainage (ES-SAGD) and Solvent-Assisted Cyclic Steam Stimulation (SA-CSS) are in situ steam-solvent recovery process to produce heavy oil and bitumen reservoirs. In ES-SAGD and SA-CSS, steam and solvent are injected into the depletion chamber within the reservoir. At the chamber edge, the steam releases its latent heat heating the oil there and solvent mixes with mobilized bitumen which then flows under gravity to the lower horizontal producer. There are many factors that influence the efficiency and rate at which oil is mobilized. One of them is the stability of the steam-oil interface which is controlled by the concentration and temperature dependencies of viscosity and density and the relative magnitudes of viscous, gravity (buoyancy), and capillary forces. In this research, the stability of the chamber interface between the vapour chamber and the bitumen at the edge of the chamber is examined. We present theoretical evidence for occurrence of such instability and conditions at which the interface is unstable. The results demonstrate that steam-solvent injection enhances the instability of the interface thus promoting greater mixing at the edge of the chamber. Consequently, the oil rate of a steam-solvent process is higher than that of a steam-only one. Therefore, there are three fundamental contributions to enhanced production by solvent-steam processes: first, the oil phase viscosity is lowered, second, the oil saturation is enhanced at the edge of the chamber, and third, the vapour-oil interface becomes more unstable which promotes more mixing at the chamber edge. The stability of the interface is also controlled by the balance between the solvent's solubility in the oil phase and the ability of the solvent to reduce the viscosity of the oil phase. The results of the stability analysis confirm the findings of laboratory experiments and field tests which demonstrate that processes that use solvent+steam yield higher production rates than that of steam alone.