Impedance measurements for the reduction of Tl+ on gramicidin-modified dioleoylphosphatidylcholine-coated mercury electrodes have been performed. The frequency dependence of the admittance data fits well to a Randles circuit, and the Warburg coefficient, sigma, and the irreversibility coefficient, p＇, can be obtained at every de potential from the frequency analysis conforming to this circuit. However, the potential dependence of the Warburg coefficient is different from the one expected for a simple electron transfer. Instead, the sigma-E data can be analyzed conforming to a mechanism including preceding and following homogeneous chemical steps to the electron transfer (CEC mechanism). In addition, from the irreversibility coefficient, p＇, a value of the standard rate constant for the electron transfer of 0.035 cm s(-1) and a potential independent value for the transfer coefficient, alpha, of close to 0.5 are obtained. The possibility that the CEC mechanism originates partly from nonlinear diffusion is considered, and the results are discussed in comparison with those given in the literature for Tl+ reduction on pure mercury.