This paper presents a simple, accurate model for determining the amount and composition of a liquid-vapor release from a pressurized tank that develops a large break above the level of the liquid. Most models commonly used by the chemical industry assume that there is thermal- and mechanical-equilibrium between the liquid- and the vapor-phase (homogeneous equilibrium models, HEM). While this assumption is valid for releases though long pipes and nozzles, we found that it overestimates the total amount released during rapid discharges through large breaks in a vessel when there is insufficient time for the mixture to become homogeneous. We derived an analytical non-homogeneous, thermal equilibrium model that accurately determines the void fraction of the mixture at the time of the release, and the quantity of a release from a pressurized container. Our model is based on equations describing the transfer of interfacial momentum between the liquid- and the vapor- phases that develop during the quick depressurization of a vessel. The model's predictions are verified by comparing them with actual measurements of the void fraction, and with the results of the RELAP5 model. Also, our model is used to determine emissions of nitrogen oxides and nitric acid in an actual rupture of a railcar tank. The results agreed with actual observations, whereas a homogeneous equilibrium model gave erroneous predictions. Published by Elsevier Science Ltd.