This paper quantifies the effect of mass transfer (suction) on wall friction for turbulent vapour flow inside ducts during condensation. Correlations are presented for the friction factor in terms of the suction rate, and are utilized in estimating the frictional pressure change in condenser tubes. These correlations are derived From the numerical integration of the velocity profile through the turbulent boundary layer, based on the Van Driest model For the Prandtl mixing length. Predictions of the frictional pressure drop during the condensation of steam in an air-cooled duct correspond to within +/- 5% of experimental measurements.