Steam deacidification is the final step in edible oils processing. Its parameters have to be optimized to ensure final product specification, avoiding neutral oil losses and high costs. To understand how the vegetable oil mixture deviates from ideality is important to optimize the purification process. In this work, the oil was considered as a complex mixture, formed by 72 components. The calculations were performed using the gamma-phi approach (Virial Equation and UNIFAC). The physical properties were estimated by recently published group contribution methods. The liquid phase presented a high positive deviation from ideality. The activities coefficients were proportional to the molecular weight and inversely to insaturations in the structures. The vapor phase behavior was close to the ideality, which was expected for the low pressure and high temperatures of the process (pressure < 5 mmHg, temperature > 220°C). The free fatty acids presented the higher deviation for the vapor phase.