Phase behaviour of C3H8-n-C4H10-heavy oil systems at high pressures and elevated temperatures has been experimentally and theoretically investigated. Experimentally, a versatile pressure-volume-temperature (PVT) system is utilized to determine the liquid-vapour phase boundary (i.e., saturation pressure lines) and swelling factors of C3H8-n-C/H10-heavy oil systems with varying compositions at high pressures up to 5030 kPa and elevated temperatures up to 396.15 K. The viscosities of the corresponding solvent(s)-saturated heavy oil systems are measured by using a customized-capillary viscometer at 298.85 K. Theoretically, the volume-translated Peng-Robinson equation of state (PR EOS) with a modified alpha function is used to model the experimental phase behaviour of C3H8-n-C/H10-heavy oil systems. Two binary interaction coefficient (BIP) correlations, respectively developed for the C3H8-heavy oil system and n-C4H10-heavy oil system, are incorporated into the volume-translated PR EOS model. The two BIP correlations together with the volume-translated PR EOS are found to be capable of predicting the phase behaviour of the C3H8-n-C4H10-heavy oil systems with a good accuracy. In addition, comparison of five commonly used mixing rules indicates that the Lobe’s mixing rule is the most appropriate to predict the viscosity of heavy oil diluted by C3H8 and/or n-C4H10.