For a deeper understanding of reinforcement mechanisms of elastomers by fillers, the geometrical surface roughness of carbon blacks was characterised by a fractal concept. The adsorption isotherms of different gases (argon, nitrogen, ethylene, ethane, propane and butane) were analysed by the yardstick-method which yields a characteristic scaling behavior between the BET-monolayer and the cross section of gases of different size. According to this study the surface can be considered to be rough with a surface fractal dimension of D-s approximate to 2,6. Hence, the variable reinforcement behavior of different carbon black types cannot be related to different surface roughness. Graphitization of furnace blacks leads to a reduction of surface roughness with a surface fractal dimension of D-s approximate to 2,3. This correlates with a reduced BET-surface area.