Steam-assisted gravity drainage (SAGD) is one of the unconventional methods used in Canada to extract heavy oil; however, a stable water-in-oil-in-water (W/O/W) emulsion is generated during production. The emulsion is then pumped into a high-temperature separator, after adding a diluent, a reverse emulsion breaker (REB), and an emulsion breaker (EB) to separate the oil and water and comply with the sales oil specifications. This work is focused on understanding the demulsification processes under high-temperature (80-120 degrees C) and high-pressure (700-900 kPa) conditions. Compressibility, crumpling film or drop detachment ratios, and interfacial tension (IFT) were used to evaluate the demulsifiers. REBs were found to decrease IFT and cause the interfacial film to be reversible. These effects showed a dependency with the temperature, and their magnitude varied with the chemistry of the REB. Effective REBs formed complexes with natural anionic surfactants present in the oil phase, weakening the irreversible film, which correlated well with lowering emulsion stability. When EB was added to the system, a synergistic effect was observed by further lowering the IFT. All of these results demonstrated that the compressibility and IFT techniques employed in this study provide valuable insights into the mechanisms of demulsification of SAGD emulsions.