In this study, a novel vapor–liquid equilibrium equation has been presented based on a simple theory for pure substances. This equation shows a relation between the non-ideal behaviors of the saturated liquid and vapor phases of pure substances, which are in equilibrium with each other. The special shape of this relation lets us to calculate the equilibrium values of the temperature, pressure or molar volume of each one of these phases. Although the idea of this theory-based equation has been raised from investigation of equilibrium PVT values of water, the empirical values of 14 substances have been used for validating the accuracy of the predictions of this equation. This comparison confirms this accuracy with RMSEmean– = 0.0254.