In this research study, in-tube condensation in the presence of air was investigated experimentally at a heat exchanger of countercurrent type for different operating conditions. The test matrix for the steady-state condition covers the range of pressures P = 1.8 to 5.5 bars, vapor Reynolds numbers Re, = 45 000 to 94 000, and inlet air mass fraction values Xi = 0 to 52%. The effect of air manifests itself by a reduction in the local heat flux and the local heat transfer coefficient. The local heat transfer coefficient is inversely proportional to the local air mass fraction. Both the local heat flux and the heat transfer coefficient vary with the system pressure and vapor mass flow rate. There is no effect of inlet superheating on the local heat flux. The film Reynolds number lies in the range of the turbulent region. Two experiments simulating loss of coolant to the secondary side of the condenser were performed, for pure steam and for an air/steam mixture. These transients show that the vapor suction rate, effective condensation length, and overall heat transfer rate are a function of the coolant boiloff rate and the air mass fraction.