Steam ejectors, or thermocompressors, are widely used in thermal desalination applications such as Multiple Effect Distillation to reclaim some of the energy discarded in the condensation of vapor from the last effect. Although several empirical models for the entrainment ratio as a metric of performance of the ejectors are available in the literature, large discrepancies are present in their predictions. In the present paper, a physical model for the performance of an ejector is developed, along with an investigation of its off-design performance. (C) 2017 Elsevier B.V. All rights reserved.