The degree of water dissolution in oil affects the flow behavior of oil-water mixtures in reservoirs and pipes. A multiphase flash algorithm was used to calculate the solubility of water in oil containing asphaltenes at various temperatures and pressures. A cubic-plus-association equation of state was used to simulate the formation of H-bonds between water molecules and polar asphaltene molecules. Experimental water solubilities in four oil samples validated that the method assumptions were accurate. The results demonstrated that the average absolute deviation between the experimental and calculated mole fractions of water in oil was less than 0.04673 in the range 452-557 K. Also, this method could explain the smooth transition from solubilized water at high temperatures to emulsified water at lower temperatures.