The Tl(Hg)/Tl+ reaction in 3 M KSCN is investigated in order to elucidate the reaction mechanism. A set of 14 impedance spectra (measurements from 0.16 to 160 kHz) is simultaneously fitted to the theoretical relations of two-and three-step reactions with partial charge transfer. The introduction of special models for the adsorption and the reaction rates allows one to diminish the number of parameters. A detailed analysis of the errors of the numerical estimation is given. The Tl(Hg)/Tl+ reaction proceeds in two effective steps with a partially charged intermediate (I-1 = 0.78 +/- 0.02). Both the partially charged Tl- and SCN--species seem to be in direct contact with Hg. The adsorption follows a Langmuir isotherm, for the chosen concentrations of the redox components (c(O) = 0.25...5 mM, c(R) = 0.2...500 mM) the coverage degree is 0.40 < theta < 0.98. The rate constants of both steps are greater than 10 cm s(-1).