A coal tar pitch and a petroleum pitch were heat treated under gas-blowing conditions (air and argon) at 300, 350, and 425 degrees C in order to investigate the chemical transformations involved. The evolution profiles of four classes of polycyclic aromatic hydrocarbons (PAH) in the toluene-soluble (TS) fractions were determined using an HPLC method. The influence of the type of gas on heat-treatment characteristics of the pitches was clearly identified. The evolution of the different classes of PAH present in the TS of a coal tar pitch to yield toluene-insoluble (TI) material is more significant and faster under air-blowing than under argon-blowing. The presence of oxygen promotes a general increase in the reactivity of the TS, especially in the case of the peri-condensed compounds. The topology of the latter strongly determines their higher increase of reactivity with oxygen. The reactivity of the cata-condensed PAH is significantly higher in the case of the petroleum pitch due to the higher presence of alkyl substituents. It is therefore concluded that the chemical composition of the raw pitches influences their behavior during the carbonization under gas-blowing conditions. The evolution of cata- and peri-condensed compounds during the heat-treatment of the pitches under gas-blowing conditions did not always fit simple first-order kinetics. A model for the overall. reaction pathway was proposed, in which p-resins constitute an intermediate product in the transformation of TS to quinoline-insoluble (QI). A good correlation between some thermal parameters obtained by TGA/DTG: and the chemical composition of the pitches expressed in terms of their HPLC compound class distribution was found.