Abstract: The resonance Raman spectra of the cationic radicals of p-phenylenediamine and its N-substituted derivatives (N-methyl, N,N-dimethyl, N,N-diethyl and N,N'-dimethyl) are reported. The spectra reveal that the radicals have a half-quinoid structure with obvious C=C and C=N double bond characteristics. The resonance Raman bands at about 1635 cm~(-1) and 1410 cm~(-1) which are caused respectively by the C=C and C=N double bonds shift to lower frequencies when methyls or ethyls are bonded to the N atom as the substituting groups are bonded directly is lost in the radicals and a partial C=N double bond is formed between this N atom and the ring C atom as the π electrons are attracted by the positive charge. The two visible absorption bands are assigned based on the changes of the resonance Ranman spectra with the alternation of the wavelength of the excitation.