1. The adsorption isotherms of ethylene and carbon dioxide were measured at 15° and 25° on carbon black heated at 3000°. The adsorption isotherms of mixtures of these gases at 25° were also studied. 2. The total adsorption isosteres in the adsorption system which was studied were linear but the phase diagram of the surface solutions constructed along these isosteres showed there was a small deviation from Raoult's law, positive for carbon dioxide and negative for the more adsorbed component-ethylene. The existence of these deviations indicated that there was a small non-uniformity of the surface of the carbon black. 3. In the system studied the “activity coefficients” of the surface solutions are linear functions of the composition of the adsorption phase with same values of the slope. For carbon black this coefficient (the value of the slope) is equal to 0.16 and in the case of evacuated surface phases it is possible to consider this coefficient as a numerical property of the non-uniformity of the surface. An analogous relation between the activity coefficients and the composition is found for the surface solutions in the adsorption of mixtures of CO2 + C2H4 and C2E4+C3H6 on active carbon. 4. The semi-empirical equations found for the adsorption isotherms of the components of binary gas mixtures permit the experimental results obtained for the adsorption of gas mixtures on carbon black and active carbon to be obtained with sufficient accuracy from the individual adsorption isotherms and the coefficient of non-uniformity of the surface.