In this work, a new and pragmatic methodology has been developed and validated to predict phase boundaries together with their types as well as solvent solubility for solvents-heavy oil/bitumen-water systems in a temperature range of 298-573 K, for which the measured data are collected from literature. More specifically, two newly modified alpha functions respectively for non-water components and water are incorporated into the Peng-Robinson equation of state (PR EOS). The binary interaction parameters (BIPs) are based on the solubility data of binary mixtures, while new BIP correlations are proposed for toluene-water pair and heavy oil/bitumen-water pairs. The BIPs for heavy oil/bitumen-water pairs (denoted as BIPHO-water) is regressed as a function of the concentration of resin and asphaltene in heavy oil/bitumen. The newly developed model is found to accurately predict the measured ALV/AL and LV/L boundaries ("A" represents the aqueous phase, "L" denotes the oleic phase, and "V" refers to the vapour phase) with an overall AARD of 4.5% and solvent solubility in the oleic phase with an overall AARD of 9.4%, respectively. It is found that addition of water may dramatically affect the phase boundaries of solvents-heavy oil/bitumen mixtures, and thus leads to a significant reduction of solvent solubility in heavy oil/bitumen, depending on solvent type, temperature, and pressure. The water molar fraction in feed plays an important role in influencing the solvent dissolution in heavy oil/bitumen in LV equilibria due to the fact that it affects the partial pressure of the solvent greatly. After adding a certain amount of water for which ALV equilibria is established, solvent dissolution in heavy oil/bitumen will not be affected by water because its subsequent addition in feed does not impose an impact on the partial pressure of the solvent. Since solvent dissolution rate in heavy oil/bitumen can be much higher in ALV equilibria than LV equilibria and a large portion of solvents can be dissolved in heavy oil/bitumen in ALV equilibria in a very narrow pressure range, effects of water must be considered at given conditions. Addition of solvents can enhance the water dissolution in heavy oil, while the enhancement is increased with carbon number of alkanes.