The in situ formation and electrochemical behaviour of a complex between gadolinium(III) and 1,10-phenanthroline (phen) are discussed. The electron withdrawing ability of phen decreases the redox potential of Gd(III). A cathodic peak at similar to-1.7 V vs. Ag/Ag(+) 10 mM, corresponds to an irreversible electron transfer reaction involving the exchange of two electrons, where the first charge transfer reaction is the rate-determining step. The diffusion coefficient and the total number of electrons were derived using the Shoup-Zsabo equation. Kinetic parameters such us k(0), alpha and i(0) are calculated. Voltammetric simulations are carried out to evaluate the accuracy of the results. A fast chemical reaction consumes the reduced species formed at similar to-1.7 V, which is then oxidised at similar to-0.1 V. The stoichiometry of the complex is determined by the Yoe and Jones method. An average value for the stability constant of the complex of (2.3 +/- 0.8) 10(5) M(-2) is obtained. (C) 2010 Elsevier B.V. All rights reserved.