The surface fractal dimension of commercial rubber-grade carbon blacks and of pyrolytic carbon blacks (CBp) was determined using nitrogen sorption data. The CBp were produced by vacuum pyrolysis of used tires at different temperatures and pressures. Commercial carbon blacks and CBp obtained at low pyrolysis pressures have a similar fractal dimension of the surface (Ds) or roughness. CBp produced at higher pyrolysis pressures have a smaller fractal dimension than commercial grades, suggesting a smoother surface. Complementary SIMS and ESCA investigations showed that the decreasing fractal dimension with increasing pyrolysis pressure is due to the formation of carbonaceous deposits on the CBp from the pyrolyzed organic material.