Pyridine insoluble fractions in the mesophase pitches derived from a decant oil (PMP-PI) and naphthalene (NMP-PI) were found to be rendered almost completely soluble in pyridine by the Diels-Alder reaction with maleic anhydride, maintaining their optical anisotropy and molecular association. Their solubility in pyridine reached up to 95 wt % by the reaction at 175-200 degrees C. Solubilized fractions were analysed to obtain their average structures, suggesting that PMP-PI consists of aromatic nuclei of peri-condensed rings connected through a small number of methylene and aryl-aryl linkages, having a molecular weight (MW) of similar to 1800, and that NMP-PI consists of oligomeric naphthalene with a large number of naphthenic groups (MW similar or equal to 1000). Such structures are basically much the same as those of their soluble fractions, although the molecules in Pls were much larger. The reaction sites in such structures for the Diels-Alder reaction are discussed.