Selective oxidation with RuO4 is used to detect differences in the molecular structure of two high-softening-point pitches obtained by the thermal treatment and air-blowing of a commercial impregnating pitch. The results of the characterization of the parent pitches and RuO4 oxidation products by chromatographic techniques have revealed differences in the condensation degree of the pitches. These differences are confirmed by the amount of CO2 evolved during the reaction, which also yields information about their reactivities.