An avionics cooling system is described which uses direct ram air cooling at low air-craft speeds and ram air driven air cycle cooling at high aircraft speeds. The air cycle cooler operates on a reverse bootstrap cycle which has numerous advantages over closed cycle refrigeration systems. It requires no electrical power during flight since all of the power is supplied by captured ram air. A small amount of electrical power is required for ground cooling. The paper includes a description of the cooling system, the component hardware, analytical predictions and experimental measurements of air cycle cooling performance. Data are provided for internal pressures, flow rates, turbine discharge temperatures and cooling capacity. The results indicate that the system cooling capacity varies from 500 W to 1600 W at sea level and from 300 W to 400 W at an altitude of 50,000 ft. The effects of flow leakage and system pressure drop are found to be acceptable for avionics pod applications.