It is known that conventional analytical techniques do not allow a complete characterization of carbon blacks. In some cases in-rubber properties of carbon blacks differ in spite of identical conventional analytical characteristics. It is shown that: structural differences are revealed by transmission electron microscopy in conjunction with advanced image analysis techniques. The methods are applied to various commercial and experimental blacks of the N 220 and N 234 class. Characteristic differences are to be seen in fractal perimeter dimension, mass density and branch length.