In an absorption cooling system, simultaneous heat and mass transfer operations take place in the absorber and generator. The performance of an absorber is of paramount importance on the coefficient of performance and the manufacturing cost of an absorption machine. Increasing heat and mass transfer coefficients in an absorber decreases the heat transfer area of the absorber, as a result of this the cost of the system can be reduced. The absorption of water vapor in aqueous solutions of lithium bromide is modelled for a falling-film, vertical-tube absorber. Heat and mass transfer coefficients are determined by calculating temperature and concentration variations at the falling film in the absorber. Then mean Nusselt and Sherwood numbers are determined to see the changes in the heat and mass transfer. Subsequently, a modular computer program has been developed for absorption systems to simulate various cycle configurations and absorber parameters. So, the effect of hot water, chilled water and cooling water inlet temperatures on the coefficients of performance, Nusselt and Sherwood numbers and the surface area of the absorber are studied with the simulation program. The results can be used to optimise the commercial absorption chillers. © 2002 Elsevier Science Ltd.