Crude oil is a mixture of light and heavy hydrocarbon components. The light components in crude oil keep the heavy components soluble in solution. However, the cooling of crude oil below its solidification point leads to the formation of a solid-wax phase. A number of thermodynamically based models have been proposed in the literature for the prediction of wax formation in both crude oils and fuels. Some of these models assume ideal liquid solution behavior. Others are a modification of the regular solution theory or the Flory-Huggins model for activity coefficients, with varying degrees of success. The absence of a proper model in the petroleum industry to describe the liquid-phase non-ideality for mixtures of hydrocarbons with large differences in size is still one of the major problems in solid - liquid equilibrium calculations. A simple methodology is proposed to develop a cubic equation of state (CEOS) for the solid phase to allow the accurate calculation of solid-liquid equilibria. The methodology is successfully applied to the Twu-Sim-Tassone (TST) cubic equation of state. The solid TST equation of state is then combined with the TST excess Helmholtz energy mixing rule to handle highly nonideal systems for the accurate prediction of solid-liquid phase equilibria.