In this study, a new and pragmatic methodology has been developed to accurately predict the viscosity for light solvents (i.e., methane, ethane, propane, n-butane, n-pentane, N-2, and CO2)-heavy oil/bitumen/water systems as a function of pressure in the temperature range of 287.9-463.4 K. The LV and ALV (L is the oleic phase, V is the vapor phase, and A is the aqueous phase) phase equilibria of the aforementioned systems are calculated using the Peng-Robinson equation of state (PR EOS) with modified alpha functions and binary interaction parameters (BIPs). The six widely used mixing rules for predicting viscosity of solvents-heavy oil/bitumen systems pertaining to vapor-liquid equilibria are compared and evaluated, while the linear mixing rule is used for hydrocarbons-water mixtures. Plus, effective density is for the first time successfully introduced into the volume-based mixing rules. The volume-based power law, weight-based power law, and weight-based Cragoe's mixing rules are found to well reproduce the viscosity for the aforementioned systems with AARDs of 15.5%, 19.0%, and 32.6%, respectively. Effective density rather than real density of dissolved gas(es) should be used for all of the volume-based mixing rules, while the adjustable parameter in the power law mixing rule has a potential to achieve high generalization if adequate measurements are made available. Although water has a lower diluting ability than other solvents in the same amount of dissolution, it can outperform methane and CO2 in diluting heavy oil/bitumen at high temperatures due to its high solubility. Addition of water can reduce or increase the viscosity of a solvents-heavy oil/bitumen mixture, depending on the ability of solvents and water to dilute heavy oil/bitumen and effects of water on the solvent dissolution. Water molar fraction in feed can exert an effect on the mixture viscosity in LV equilibria through affecting the solvent dissolution but cannot impose an impact on the mixture viscosity at ALV equilibria.