Reservoir simulation and research activities of heavy oil production processes (steam-assisted gravity drainage, vapour extraction, or the steam/solvent combined hybrid process) require accurate values of equilibrium constants (K-values) at elevated temperatures. However, such data are rarely available in open literature either because of the temperature limitation of pressure/volume/temperature (PVT) equipment or difficulties in performing high-temperature three-phase vapour/liquid equilibrium (VLE) experiments. In this paper-with the assistance of a state-of-the-art fully visual PVT cell that was specifically developed for heavy viscous oils, is capable of performing fluid-phase-behaviour studies at temperatures up to 250 degrees C, and that effectively mixes samples with viscosity up to 1,000 cp at test temperature-a heavy-oil/solvent/steam equilibrium case study is presented. The heavy oil was mixed with solvent and water at a specified weight ratio, temperature, and pressure in the heavy-oil PVT cell. After achieving equilibrium, the volumes of vapour, liquid, and water phases were measured. The samples from each phase were drawn for compositional analysis using gas chromatography (GC), and K-values of the hydrocarbon components were examined. The component compositions from this experiment were used for equation-of-state (EOS) model tuning. This case study shows that with proper care, the EOS can be characterized and used to match with experimental data for heavy oil.