Standard electrode reaction rate constants for the one-electron reduction of Eu(CF3SO3)(3) on mercury in different concentrations of the supporting electrolyte tetrabutylammonium perchlorate in dimethylsulfoxide have been measured. Two techniques (i) ac polarography on the dropping as well as on the hanging mercury electrode and (ii) cyclic voltammetry in a two electrode system were employed. The evaluation of the ac data was carried out by the Randles and the Sluyters methods. The reaction rates from cyclic voltammetry were extracted via a simulation program. Physical properties such as the viscosities, resistivities and the double layer capacitances necessary for the evaluation of the data are given. The decrease in the standard electrode reaction rates with increasing supporting electrolyte concentrations was accounted for in the Frumkin model.