The purpose of incorporating a jet ejector into an absorption system is to improve the preabsorption of the refrigerant coming from the evaporator by the weak solution, i.e., to improve the overall absorption process. The mixing process in the jet ejector is very intensive as a result of spray generation of the liquid phase and of extensive subcooling of the weak solution in the solution heat exchanger. To facilitate the design of a jet ejector for absorption machines, a numerical model of simultaneous mass and heat transfers between the liquid and gas phases in the jet ejector was developed. The steady-state model was based on unidimensional balance equations of mass, energy and momentum for the liquid and gas phases. Polynomial correlations were employed to calculate the thermodynamic properties. On the basis of the developed model, we studied the following design parameters of the jet ejector as functions of the length and the cone angle of the diffuser: pressure recovery, temperature and concentration of the refrigerant in the solution, and velocities of the gas and liquid drops. The parametric study also involved examination and ways of augmentation of the mass transfer process in the diffuser with the ultimate aim of designing a compact and efficient unit. The calculations were performed for a mixture of the refrigerant R125 (pentafluoroethane) with organic absorbents.