The isotherms of adsorption W(p) on different types of microporous carbons and their derivatives are analyzed. It is shown that all isotherms can be fitted by a modified Freundlich (FR) equation : W(p) = kp(alpha) + k’ taking into account the limits of the micropore domain. The physical meaning of the exponent alpha = RT/betaE0 arising from the thermodynamic approach of Dubinin is discussed on the basis of the molecular theory of micropore filling. It follows that E0 contains implicitly the fractal dimension D.