The acid doping process of the mixtures of N,N'-diphenyl-1,4-phenylenediamine (DPPD) and N,N'-diphenyl-1,4-quinonediimine (DPQD), reduced and oxidized phenyl-capped aniline dimers respectively, has been studied in detail by UV-vis-NIR and ESR spectroscopies. The addition of the acid to the mixtures of DPPD and DPQD leads to the formation of one type of radicals by intermolecular interactions. Full conversion of the aniline dimer molecules to the radical form is achievable only in stoichiometric mixtures of DPPD and DPQD. Concentration of radical species may change over time according to processes involving hydrolysis. The mechanisms of the acid doping of the phenyl-capped dimer process and subsequent side processes are proposed.