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 used to determine the liquid/vapour phase boundary (i.e., saturation-pressure lines) and swelling factors of C3H8/n-C4H10/heavy-oil systems with varying compositions at high pressures up to 5030.0 kPa and elevated temperatures up to 396.15 K. During the experiments, heavy oil is added continuously into the solvent and the injection process can be terminated within a short period. No noticeable asphaltene precipitation has been observed throughout the measurements for the four mixtures. 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) (Peng and Robinson 1976) with a modified alpha function is used to model the experimental phase behaviour of C3H8/n-C4H10/heavy-oil systems. Two binary-interaction-parameter (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 saturation pressures and swelling factors of the C3H8/n-C4H10/heavy-oil systems with a good accuracy, although the prediction accuracy is reduced at temperatures close to the critical temperature of a pure solvent. In addition, comparison of five commonly used mixing rules indicates that the Lobe's mixing rule (Lobe 1973) is more appropriate to predict the viscosity of heavy oil diluted by C3H8 and/or n-C4H10.