Abstract A model for predicting heat and mass transfer and pressure drops occurring inside a tube-in-tube refrigerant cooled absorber (RCA) is developed. This heat exchanger is used as a low pressure absorber in a half-effect water-ammonia absorption chiller. The two-phase stream, resulting from the mixing at low pressure of the weak solution coming from the generator and the refrigerant leaving the evaporator, flows in the tube-in-tube annulus, while the stream of condensed refrigerant, throttled to a pressure level intermediate between condenser and evaporator pressures, flows counter-current in the internal tube. The RCA model is validated by comparing overall heat transfer duty, pressure drops on each stream and temperature axial profile in the annulus with experimental data. Results are in reasonable agreement with experiments in most of the analyzed cases, although some deviations exist in off-design operations. A sensitivity analysis of the model has shown that the measurement uncertainty of the inputs to the model does not affect the validity of the results.