In this paper, a mathematical model for simultaneous heat and mass transfer in an ammonia-water, falling film type of absorber is presented. A nondimensional analysis shows that the overall absorption process can be decomposed into two basic processes: absorption owing to the subcooling of the liquid solution, and absorption owing to the cooling from the wall. Empirical correlations for heat and mass transfer coefficients were developed for film flows with short mixing distances. Numerical results show that both heat and mass coefficients are affected by the subcooling of the inlet solution as well as by the him thickness.