This paper describes results of a design-oriented model for two-phase jet pumps and ejectors for refrigeration and thermal management aerospace and terrestrial applications. The primary motivation for the work is the development of a reliable but simple design methodology that captures the important flow physics while being sufficiently fast in order to reduce the design cycle time. This work is particularly relevant to the flow boiling test facility under development at the NASA Marshall Space Flight Center as it allows rapid design optimization of the jet pump as well as the integrated system. The results presented in the paper show optimal geometric area ratio as well as system state point information as a function of the inlet states and entrainment ratio. Qualitative agreement with single-phase ejector performance is predicted, lending additional confidence in the results. Copyright (C) 2000 John Wiley & Sons, Ltd.