The determination of the chain-structure of carbon black by mercury porosimetry is described. It is shown that each type of black has its own porosity curve, depending on specific surface area and degree of structure. An absolute measure for the degree of structure is proposed: The “Structure Index”, calculated from the break point in the porosity curve, the specific surface area and the density of the black. Structure Index for structureless blacks is 1.0 – 1.1, for low structure blacks 1.7 – 1.9, for normal structure blacks 2.3 – 2.4 and for high structure blacks 2.7 – 2.8.