This paper reports the development and application of a mathematical model for the prediction of the low-pressure-vaporization (LPV) process of free water. The study is focused on defining clearly the two stages of the LPV process (before and after the so-called flash point) and on evaluating their contribution for the overall transient evolution of the relevant parameters. The physical domain was divided into two control volumes: the first one contains the mass of free water, ideally assumed at a uniform temperature; the second one includes the volume of the vaporization chamber above the water free surface, the condenser and the vacuum pump. The governing differential equations of the model were solved by the Euler method using the EES-Engineering Equation Solver. The results obtained show that the mathematical model describes the complete process of low-pressure-vaporization of free water, giving evidence to the weight of the first stage on the process transient evolution. (C) 2012 Elsevier Ltd. All rights reserved.