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Measurements are described of local heat transfer by free convection under steady state conditions around a 2-in o.d. circular cylinder mounted with its axis horizontal in an isothermal large enclosure, at several levels of wall temperature ranging from about 20°F below ambient, to about 80°F above ambient. Helium was injected through the porous wall of the cylinder into the boundary layer at a uniform rate per unit area of outside cylinder surface, in the range 1.00–7.50 lbm/h ft2. Analysis of the results showed that when the convective heat flux is zero, the wall temperature, in this case termed the adiabatic wall temperature, was higher than ambient temperature by up to 190°F, depending upon the location around the cylinder, and the helium injection rate. This phenomenon, so far unreported in free convection, is similar to the findings in references [1, 2], and hence is due to the diffusion thermo-effect. The local heat-transfer coefficient was defined in terms of the adiabatic wall temperature, and determined to be essentially independent of the temperature difference between wall and ambient. At the lower stagnation point, its value increased rapidly with injection, but then levelled off to an almost constant value when the injection rate exceeded about 2 lbm/h ft2. At a given injection rate, the heat-transfer coefficient decreased from a maximum value at the lower stagnation point to a minimum value at the upper stagnation point. At a given location on the cylinder circumference, the adiabatic wall temperature increased steadily as the injection rate increased. At a given injection rate, the adiabatic wall temperature increased from a minimum value at the lower stagnation point to a maximum value at the upper stagnation point. The body forces at work in the binary boundary layer are discussed in their importance on heat transfer.