The major irreversibilities in absorption heat pump technology are associated with the transfer of heat into and out from the machine. By modeling only these irreversibilities, a model is formulated that provides significant insight into the thermodynamics of such cycles. In particular, the shape of the COP curve as a function of heat input temperature does not follow the intuition developed from a fully reversible model. Such a fully reversible model predicts that the COP should rise with the heat input temperature. Instead, the COP of an actual absorption machine falls with a rise in heat input temperature. The explanation of this detail clarifies a number of aspects of the technology and provides insight into such questions as what performance can be expected from future advanced cycles. The model developed here is internally reversible but includes the irreversibilities associated with the external heat transfer processes. Observations made from the model are compared against experience with actual absorption systems to arrive at several generalizations that are useful for absorption machine design.