Differential scanning calorimetry (DSC) and thermogravimetric analysis measurements were used to study the influence of the acidic comonomers acrylic acid, methacrylic acid, and itaconic acid on the exothermic reactions occurring during the heat treatment of acrylonitrile copolymers under air atmosphere. The presence of these reactions was noticed in the DSC exotherms of the copolymers, which appear as doublets. These doublets were resolved into their constituent peaks, arising due to the occurrence of oxidative and cyclization reactions, and the area under the resolved peaks was considered as the extent of the reaction. The comonomers under study were found to enhance the secondary oxidation reactions and retard the extent of cyclization reactions. The degree of cyclization decreases abruptly with the increase in the comonomer content beyond similar or equal to 2 mol %.