A thermodynamic analysis of the heating performance of air cycle heat pumps with and without liquid water injected at the compressor inlet was conducted. Two basic cycles were considered: a simple cycle and a regenerative cycle. Various levels of compressor and expander isentropic efficiency and heat-exchanger pressure drop were assumed. Calculations predict that water injection can substantially improve the efficiency of an ideal air-cycle heat pump, as much as doubling the coefficient of performance at low pressure ratios. However, with an assumed 5% pressure drop and component efficiencies as high as 0.95, no significant improvement in efficiency can be derived from water injection with either cycle. The injection of water drastically reduces the heating output per unit mass of air circulated.