A theoretical analysis of the thermodynamics of the oxidation–reduction reaction steps associated with proton-coupled electron transfer is presented for aniline and phenylenediamine isomers. In this context, the radical scavenging ability of the parent molecules and their ionic and/or radical species is compared. The acidity constants evaluated for heterolytic N–H bond decay in an aqueous environment and corresponding electrochemical oxidation potentials, as adapted to the standard hydrogen electrode, were used for the construction of a Pourbaix diagram (electrochemical potential vs. pH). Based on this diagram, the chemically relevant transformation routes for the corresponding radical cations were identified. Finally, the thermodynamics of model proton-coupled electron transfer reactions with hydrogen peroxide are considered. The theoretical data presented are compared and correlated with experimental results, published in the literature. The results obtained allow an estimation to be made of the oxidation mechanisms occurring in polymer matrices, including those containing phenylenediamine antioxidants, in synthetic organic reactions or in processes related to tertiary wastewater purification.