Carbon dioxide (CO2) was shown as a promising alternative for oil sands tailings treatment with economical and environmental benefits. This study aims to understand the role of CO2 addition in densification of oil sands tailings. In this study, CO2 was pressurized into two industrial whole tailings provided by Syncrude Canada Ltd. and Canadian Natural Resource Ltd. The optimal initial settling rate, supernatant clarity and solids content of sediment were achieved at a CO2 partial pressure of about 100 kPa. The improvement on densification of oil sands tailings by CO2 was mainly attributed to pH reduction under various CO2 partial pressures. The zeta potential of fines became less negative with decreasing pH, enhancing coagulation of fine solids. On the other hand, CO2 bubbles formed by dissolved gas under supersaturation pressure led to a less dear supernatant by disturbing the formed sediments. Supersaturation with nitrogen was applied to the oil sands tailings to verify the influence of dissolved gas on solids sedimentation. The limit to CO2 sequestration by oil sands tailings was experimentally evaluated.