The objectives of this research are the prediction of the thermodynamic behavior of a multi-stage intercooled reciprocating compressor and its progressive loss of performance due to leakage. A theoretical model is developed to simulate the thermodynamics of the compressor system and the lubricating condition and wear of the piston ring pack for a multi-stage intercooled reciprocating compressor. A first law of thermodynamics approach is used to determine the thermodynamic properties of the gas inside the cylinders, the intercoolers and the inlet and discharge manifold. The compressor valves are modeled as single degree-of-freedom, spring-mass=damper systems. The flows through the valves are calculated based on the steady flow equations for equivalent orifices. The lubricating condition of the piston ring pack are determined on the basis of hydrodynamic lubrication theory. The wear of the piston rings is assumed to occur when the hydrodynamic oil film between the piston ring and cylinder bore breaks down. Based on the theoretical model, a computer program is developed. This program is tested on an Ingersoll-Rand Model 242, two stage aircooled reciprocating air compressor. The comparison of the experimental values of the pressure variations in the first cylinder with the value predicted by the computer program shows a reasonable match. The computer program predicts the pressure, temperature and mass flow rates for each cylinder and the intercooler. Also predicted is the wear rate of each piston ring. The progressive loss in the compressor mass discharge, and hence the loss in its performance, is determined by calculating the leakage losses several times, updating the leakage area each time based on the wear rate of the piston rings. The result shows a drop of about 15 percent in the discharge rate of the Model 242 compressor after 8000 hours of running time. / Master of Science