Mercury porosimetry has been used to characterize the macroporosity of several carbon materials activated by steam or carbon dioxide and their surface fractal dimension is determined. The first part is devoted to the analysis of the differential volume versus pressure curves in order to distinguish between mechanical effects resulting from the mercury pressure and true pore filling. It is shown, in the second part, that the macropore volume is not strongly affected by the activation process, but it depends mainly on the initial texture of the carbon. On the other hand, the fractal dimension varies largely with the activation conditions and burn-off. The experimental results suggest that the fractal dimension tends to increase with the degree of burn-off in steam, whereas the opposite occurs during activation by carbon dioxide. These observations are discussed in relation to the mechanisms of activation.