Ejectors are compact mechanical devices that utilize the expansion of a high-pressure primary fluid to entrain and compress a low-pressure secondary fluid by means of momentum transfer between the two streams of fluid. Condensing ejectors feature both momentum and heat transfer through the interaction between the vapor and liquid streams. This paper focuses on the thermodynamic analysis and computational modeling of the vapor-liquid interactions in two specific types of condensing ejectors — one with primary liquid and secondary vapor flows, and the other with primary vapor and secondary liquid flows. Control volume analysis of the mass, momentum, and energy balance in each phase and across the interface based on one-dimensional (1D) slug flow was conducted for the ejectors. The friction coefficient on the inner wall of the ejectors and the interfacial heat transfer coefficient are identified as controlling parameters for the ejector performance in terms of pressure and temperature distributions along the axis. The 1D slug flow models for both types of ejectors are validated with published results in the literature. After validation, in-depth parametric studies are presented to scrutinize the effects of major geometric parameters and operating conditions on the performance of both types of ejectors.