In petroleum reservoir fluids, wax is mainly formed from n-alkanes (normal alkanes) at specific thermodynamic conditions. Through the wax formation phenomenon, in addition to the solid-liquid phase transition, some n-alkanes undergo an order-disorder solid-solid phase transition. These phase transitions appear in the SL (Solid-Liquid) fugacity ratio of pure component in the forms of phase change enthalpy and heat capacity. In this work, a new predictive approach with the use of new generalized heat capacity correlations for liquid, disorder and order solid phases has been presented to calculate the SL fugacity ratio. These correlations have been obtained and evaluated by using 1912 data points of pure n-alkanes according to the group contribution method. Additionally, the effective parameters in the SL fugacity ratio calculation have been obtained by an exhaustive sensitivity analysis. The results show the important contributing parameters in the SL fugacity ratio are related to the phase changes' enthalpies. Finally, the simplified model with the new modified P-UNIQUAC (Predictive UNIQUAC) has been evaluated by the experimental data of wax formation with an AAD% (Average Absolute Deviation percent) of 2.81%. (C) 2012 Elsevier Ltd. All rights reserved.