The transfer of pyridinium ion across the water/nitrobenzene interface has been studied by the methods of cyclic voltammetr and ac-impedance measurements. The diffusion coefficients of these species in both media. D-w,D-pyH+ = 4.4 x 10(-5) and D-NB,D-pyH + = 2.16 x 10(-5) cm(2) s(-1), as well as the diffusion coefficient of pyridine in the organic phase, D-NB,D-py, 3.05 x 10(-7) cm(2) s(-1), have been calculated. The standard potential of pyridinium ion transfer, and the corresponding free energy change and its distribution coefficient between the two phases, have been reported by means of cyclic voltammetr. Impedance measurements resulted in an arc in Nyquist diagrams representing the transfer of pyridinium ion while a corresponding Zarc element in equivalent circuit illustrates the transfer. The apparent rate constant and the transfer coefficient have been calculated by means of two methods.