The experimental setup Cyrène has been designed and operated by CEA in partnership with CNES and IMFT in order to provide future designers of two-phase ammonia environmental control systems with adequate knowledge and data. The whole loop (esp. boiler, visualization boxes and condensers) is constructed with 4.8 mm aluminum circular pipe. Ammonia mass flow rate and quality values range up to 0.24 g/s and 0.8 respectively. Results presented in this paper have been obtained in three parabolic flights performed in May 1997 on the CNES Airbus A300.Flow patterns have been identified from high speed video recordings at 500 frames/second through two visualization test sections downstream from the evaporator outlet. Four different flow configurations have been characterized versus the mass flow rate and the quality or versus the superficial velocities of the liquid and the vapor phases: bubbly, slug, slug-annular and annular flows.Bubbly flow is seldom observed. It only occurs for void fraction smaller than 0.1 or quality smaller than 0.002. Coalescence between bubbles is very strong due to the ammonia physical properties and the low mass flow rates, leading to a rapid transition to slug flow. For an increase in the quality, slug flow is observed and also annular flow qualities greater than 0.7. The results with boiling ammonia are compared to flow pattern maps obtained by other authors with different fluids.From the flow pictures, and after image processing, the bubble velocities have been measured. They are well predicted by a drift-flux model. They can be calculated versus the mixture velocity. These results are similar to those previously obtained with adiabatic air-water flows.Four local heat transfer coefficients have been determined on the boiler. Data points presented concern 3 pressures (0.9, 1.3 and 1.8 MPa), 3 outlet qualities (0.3, 0.6, 0.8), 3 heat flux (1.7, 3.3 and 5.0 W.cm-2) and 3 gravity levels (microgravity, 1 g and 1.8 g). A correlation for saturated boiling heat transfer coefficient is given, based on 567 data points.