Abstract Coinjecting solvent with steam under steam assisted gravity drainage (SAGD) process to reduce the required steam for production has gained importance in recent years. An extensive 2D simulation study to better understand the drainage mechanism of steam with solvent coinjection in the SAGD process shows that the relative condensation times of solvent and steam result in different films of gas solvent, liquid solvent, and water along the fluid interface. There is an optimal solvent type and concentration ratio range for a particular reservoir and operating condition. The solvent stays in or approaches the liquid phase ahead of steam along the fluid interface. Coinjecting the solvent at low concentration ratios can take advantage of the solvent solubility effect without losing too much heat effect from steam. In addition, this study indicates coinjection of suitable solvent mixtures leads to better performance than injection of pure solvent in the field.