The TEMP 2A-3 experiment was the first flight of a mechanically pumped two-phase ammonia thermal control system. This proof-of-concept mission was successfully flown on the STS-46 Shuttle flight in August 1992. The TEMP experiment performed well and all mission objectives were met. Valuable data has been obtained on two-phase pressure losses, heat transfer coefficients, and fluid management techniques in a micro-gravity environment. Overall temperature control results were excellent and within expected ranges. However, there were substantially more instabilities in the flow when compared with ground test data. Fortunately, the instabilities did not severely affect system operation. A description of the TEMP 2A-3 experiment is given and a comparison of the ground thermal vacuum and flight test data is presented.