The free convection heat and mass transfer about a porous horizontal cylinder is investigated experimentally. Various gases, which are either preheated or precooled, are injected through the porous cylinder into an otherwise quiescent air environment. The gases include hydrogen, helium, carbon dioxide, and Freon-12; the first two of these have molecular weights less than that of air while the latter two have molecular weights greater than that of air. It is found that for the condition of no net heat transfer at the surface, the temperature of the surface differs from the temperature of the environment. The existence of the aforementioned adiabatic wall temperature is due to the diffusion thermo effect. When hydrogen or helium is the injected gas, the adiabatic wall temperature exceeds the ambient temperature by as much as 100F. On the other hand, when carbon dioxide or Freon-12 is the injected gas, the deviation between adiabatic wall and ambient temperatures is both much smaller and of opposite sign relative to that noted previously. Nusselt number results are also reported and discussed. Comparisons of the experimental findings with those of analysis are made whenever possible.