Using a new mixing rule, mutual solubility of water-hydrocarbon systems is calculated with three cubic equations of state (EOS). Based on the van der Waals' mixing rule for the a-parameter of the cubic EOS, a new mixing rule is developed for considering the hydrophobic interactions in the water-rich phase. This mixing rule is not quadratic; hence it does not satisfy the low density boundary condition. However, when it is used with the cubic EOS, the quantitative description of the mutual solubility of hydrocarbons and water is quite satisfactory for a wide range of temperature. For each binary water-hydrocarbon system, three or four binary parameters are required to represent the liquid-liquid equilibria. The average absolute error for calculating water solubility or hydrocarbon solubility is within 7%.