Appreciable N-14/N-15 equilibrium isotope effect has been observed for the electron-transfer process between N,N,N ' ,N ' -tetramethyl-p-phenylenediamine (TMPD) and its radical cation perchlorate (TMPD+.ClCO4-, Wurster's blue). The equilibrium constant for the reaction, TMPD + [ N-15(2)]TMPD+. reversible arrow TMPD+. + [N-15(2)]-TMPD, was determined to be 0.85 +/-0.05 at 25 degreesC in acetonitrile by electron spin resonance (ESR) analysis of the line-broadening effect of Wurster's blue in the presence of its parent neutral molecule. Redox potentials of TMPD and [N-15(2)]TMPD were determined by cyclic voltammetry to be E-o = 0.107 and 0.103 +/- 0.002 V vs SCE respectively, corresponding to K = 0.86 +/- 0.06 for the reaction. Comparison of the infrared and Raman spectra of TMPD, [N-15(2)]TMPD and their radical cations revealed significant vibrational frequency shifts caused by the heavy-atom substitution and radical cation formation, from which the free energy change for the electron-transfer process was estimated to be 300 J mol(-1), corresponding to K = 0.89. These results demonstrate that N-15-substitution of TMPD decreases appreciably the ionization potential of the molecule, making it easier to lose an electron forming the corresponding radical cation in solution.