A method based on filtràtion test is used for the characterization of impregnating pitches. Pitches with the same KS, but with increasing degree of thermal evolution, issued either from crude or filtered coal tar, underwent the filtration test. They were characterized by Transmission Electron Microscopy and Optical Microscopy. The results obtained show that the filtrability index decreases from pitches with only γ resins (toluene soluble), to pitches with only various α resins such as clean mesophase spheres (anthracene oil sensitive but quinoline insoluble) or edge to edge associations (anthracene-oil and quinoline insoluble). The filtrability is minimum in pitches where the edge to edge associations are quinoline soluble (i.e., β resins). Thus the main limiting factor for the filtration test is the presence of α or β edge to edge associations, the β ones being more efficient than the α ones for blocking the filter. The β edge to edge assocations result from an evolution under stresses without intervention of temperature. The α edge to edge associations result from a normal thermal evolution.