The global climate change problem has focused renewed attention on technologies that have minimal or zero ozone-depletion and global warming potentials. Absorption heat pumps are environmentally sound and energy-efficient alternatives to CFC-based systems in the area of space-conditioning. In ammonia-water cycles, the absorber presents a bottleneck to heat and mass transfer and governs system viability. These inherently coupled processes have not been understood well, leading to poor designs and often oversized heat and mass exchangers. Therefore, a study of ammonia-water heat and mass transfer in the absorber is proposed. A test facility replicating an entire absorption system is developed and provides realistic operating conditions to the absorber, while heat and mass transfer rates are analyzed in detail. The absorber consists of a tube bank with four columns of six 9.5 mm tubes installed in an outer shell that allows heat and mass transfer measurements and optical access for flow visualization. Tests are being conducted over a wide range of solution concentrations, pressures and flow rates. Mass, species and enthalpy balances are established for each component in the system. Component level measurements at the absorber are used to determine heat transfer rates, overall thermal resistances, and solution-side heat transfer coefficients. High-speed video recordings of the falling-film process are used to guide the interpretation of test results. Component level measurements and the results from flow visualization are used to develop empirical models for heat and mass transfer coefficients in the absorber as a function of solution flow rate, concentration, and operating pressure. The resulting models provide tools for the design of binary-fluid heat and mass exchangers.