Wax precipitation is one of the most important flow assurance problems that cause a decrease of oil production rates, an increase of energy consumption and failure of facilities. A very simple and accurate model can be used as a fundamental tool to predict and prevent solid precipitation at different conditions. An extension of the Won (1986) model is presented that calculates the Wax Appearance Temperature (WAT) considering the temperature dependence of the solubility parameters of the individual components in the liquid and solid phases as well as the molar volumes of the individual components. In addition to predict WAT, the liquid and the solid composition in equilibrium are estimated without having available the Paraffins, Naphtenes and Aromatics (PNA) analysis. Results are compared for 18 oils from literature data. Regarding the experimental data it is enough to know only the molar fraction and the molecular weight of each component. Even if the oil composition is known only up to C7+ fraction the extension of residue is possible through Pedersen et al. (1989) method. A percentile absolute average deviation of about 2% to 3% for WAT was obtained.