A detailed analytical study has been carried out to examine the effects of buoyancy in a boundary layer where there is mass injection through a porous surface. Specific consideration is given to helium injection into air in the stagnation-point region of a horizontal cylinder. Mass and energy transport by thermal diffusion and diffusion thermo are also included in the analysis. It is found that both the transpiration-induced buoyancy and the diffusional transports play a decisive role in determining the heat transfer when the wall-to-stream temperature ratio (Tw/T∞) is only moderately different from unity. In particular, when Tw/T∞ > 1, the tendency of the transpiration-induced buoyancy to increase the heat transfer is opposed by the action of diffusion thermo. For the condition of the adiabatic wall, the wall temperature may exceed the stream temperature by an appreciable amount; this is due to diffusion thermo. The predictions of the analysis are compared with available experimental data.