A two-phase loop to measure the condensation heat transfer coefficient of R-410A was designed, built, and calibrated. The test section was a 3 m (10 ft) long horizontal, rectangular brass (63% Cu, 37% Zn by mass) tube 12.7 mm (0.5 in.) wide and 25.4 mm (1.0 in.) high. Uncertainty analysis was performed for each set of collected data. Any experimental data with an uncertainty greater than 10% was discarded. Also, any experimental data that failed to satisfy the energy balance for the test section within 5% was discarded. Over three hundred experimental runs were made. They covered a mass flux range of 30 to 200 kg/(s·m2) (2.2×104to 1.48×105lbm/h·ft2), a refrigerant saturation temperature range of 7.8 to 36.7°C (46 to 98°F), and a saturation pressure range of 1.02 to 2.23 MPa (147.4 to 323.4 psi). From this experimental database, 173 sets of data, which satisfied both the heat balance requirement and uncertainty requirement, were selected for further analysis. This study focused primarily on measurement and prediction of condensation heat transfer coefficients and the relationship between heat transfer coefficients and two-phase flow regimes. Flow pattern studies showed that the condensation was predominantly in the wavy stratified flow regime at a mass flux of less than 200 kg/(s·m2). For higher mass flux rates, the condensation was in the annular flow regime. The regionally averaged heat transfer coefficient for condensing R-410A in a rectangular channel decreased with a decrease in quality, and increased with an increase in mass flux. Using the experimental database a correlation was developed to predict the regionally averaged condensation heat transfer coefficient. In the same experimental study, the effect of oil contamination on the heat transfer performance of condensing R-410A was also investigated. Tests were conducted with two different oil concentrations (1.26% and 2.51% by mass). Results indicated that the presence of oil decreased the condensation heat transfer coefficient by 10 to 20%. © 1999 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.