A miniature free-piston engine air compressor is under development as a portable compact power supply. The performance of the air compressor side of the device was measured by separating the compressor portion from the free-piston engine and bench testing it independently. The compressor was operated over a speed range of 1000 to 10,000 cycles per minute, and output mass flow of air was measured with a load pressure range of 0 to 415 kPa (0 to 60 psi) above ambient. Two thermodynamic models of the air compressor were created. The first divided the compressor cycle into four ideal gas processes. The second model simulated the cycle through many small steps, calculating the thermodynamic states iteratively using conservation of energy for an open system. Valve effects and leakage losses were included in both models. The models were validated by comparing the results with experimental data.