In this paper, techniques have been developed to experimentally and theoretically study phase behaviour of CH4-C3H8-heavy oil systems under reservoir conditions. Experimentally, PVT tests are performed to measure saturation pressures and swelling factors of a C3H8-heavy oil system and a CH4-C3H8-heavy oil system under reservoir conditions, respectively. Theoretically, the volume-translated Peng-Robinson equation of state (PR-EOS) together with the modified alpha function has been used to model phase behaviour of the above solvent(s)-heavy oil systems. The binary interaction parameter (BIP) correlation is developed for CH4-heavy oil systems based on the literature data. As for the CH4-C3H8-heavy oil system with a fixed composition, both saturation pressure and swelling factor increase with temperature. The PR-EOS together with the modified alpha function and the newly developed BIP correlation is able to predict the saturation pressures and swelling factors of the CH4-C3H8-heavy oil systems with average absolute deviation of 11.89% and 2.22%, respectively.