This paper describes an experimental investigation, supported by analysis, of free convection in an air-water vapor boundary layer at the stagnation point of a horizontal cylinder. A two-component boundary layer is created by the effusion of water vapor from the porous surface of the cylinder; the ambient gas is pure air. The results of both experiment and analysis indicate that the heat transfer decreases as the surface mass-transfer rate increases. At moderate blowing rates, there is good agreement between the experimentally- and analytically-determined Nusselt numbers. At higher mass-transfer rates, the data lie above the analytical predictions by about 25 per cent. This departure is attributed to a fluctuating motion in the boundary layer.