The energetic surface heterogeneity of carbon black has been investigated by volumetric gas adsorption using ethene as a molecular probe. From adsorption isotherms of ethene the energy distribution functions of adsorption sites were calculated using an iterative analytical integration method, A comparison of the integral distribution function with thermodynamic values of the isosteric heat of adsorption as a function of surface coverage confirms the evaluation procedure. Four discrete species of energetic adsorption sites have been identified in all furnace black grades investigated, It is shown that the fraction of high energy sites of furnace blacks decreases with increasing primary particle size.