Future large spacecraft will require thermal control systems capable of transporting large amount of heat over long distances. Two phase fluid loops are being developed in order to satisfy these requirements.A 600 W mechanically pumped two phase elementary loop, using ammonia as the working fluid, has been built and tested at Aerospatiale-Cannes. The general operating principles of the loop and the thermal performances of the horizontal smooth one-tube condenser, uniformly cooled by water have been studied.No starting difficulties, control problems or flow instabilities have occured, and the tests have demonstrated good operating conditions. Condensation average heat transfer coefficients of 5000 to 8000 W/m2.°C have been obtained.A condensation model, previously developped, has given the theoretical two phase flow patterns corresponding to the test cases, and the predicted thermal performances for stratified and annular flows.The experimental condensation lengths and average heat transfer coefficients have been compared with the predictions. The theoretical values have been found to be in good agreement (±10%) with test results for annular flow. For stratified flow, the comparison between experimental and calculated values have shown a systematic difference of about 30%.