The oil sands industry has implemented CT (Composite or Consolidated Tailings) process to manage the fine tailings produced during extraction. The co-disposal approach using CT placed beneath MFT (mature fine tailings) was proposed to improve the CT release water quality and the solids densification rates of both CT and MFT. This research used laboratory-scale static column mesocosms to study the physical, chemical, and microbiological changes occurring over time in this CT beneath MFT deposition. The results were compared to those in the control systems. The study showed that, over 1 year of incubation, the release water composition had been altered by reduction in the concentrations of calcium and sulphate ions, and the electrical conductivity; the MFT had steady densification rate; and the CT had a densification rate comparable to the control system. These results indicate that the CT beneath MFT deposition may be a beneficial disposal scheme for oil sands tailings management.