Water dissolution in crude oil becomes significant at temperatures > 150-degrees-C, and at 250-degrees-C, water solubilities in heavy crudes are almost-equal-to 40 mol%. Dissolved water acts as a low-viscosity solvent that reduces oil-phase viscosity. This phenomenon has been considered in thermal recovery simulations but has never been substantiated. In this work, the effect of water on viscosity was measured for four crude samples with gravities ranging from 0.97 to 1.03 g/cm3. At the highest experimental temperature of 286-degrees-C, viscosities of water-saturated samples were about one-half those of water-free counterparts. The viscosity reduction, although quite significant, was not as pronounced as the drop estimated by viscosity mixing rules used for hydrocarbon systems. While a log mixing rule or a one-quarter power mixing rule overestimated viscosity effects, a mole-fraction-weighted average of oil and water viscosities matched the experimental data. A possible explanation for failure of the log mixing rule is that the water dissolved in the oil exists not as monomers but as hydrogen-bonded clusters. We find good agreement with experiment when the mole fraction of water clusters, calculated from a statistical mechanics based theory, is used in the log mixing rule.