Abstract Membrane-based liquid desiccant system is a promising technology for efficient humidity control. Also, in comparison to systems using conventional desiccants, ionic liquid (IL) desiccants enable increased system operational envelope and efficiency. In this study, a finite difference numerical model is developed for an IL-based counter and cross flow internally cooled polymer heat and mass exchanger (i.e. absorber). A super-hydrophobic membrane separates the IL desiccant and air flows while allowing moisture transfer from air to IL. The numerical model determines the outlet conditions of all three absorber fluids (water, desiccant, and air), establishing the absorber heat and mass transfer performance. The model was compared with the experimental data obtained from an IL desiccant absorber under a wide variety of water, desiccant, and air inlet conditions. The maximum discrepancy between the model predictions and experimental data for the air exit temperature, air exit relative humidity, cooling water exit temperature, and solution exit temperature are 4%, 9%, 5%, and 2%, respectively. A comprehensive parametric study is then conducted to evaluate the sensitivity of the absorber performance to different input conditions. This highly accurate model and parametric study of a membrane-based absorber can be utilized in design and performance analysis of emerging liquid desiccant dehumidification and separate sensible and latent cooling (SSLC) systems.