In the present paper, the electrochemical behavior of the Eu(III)/Eu(II) redox couple has been investigated at conductive diamond thin film electrodes. Measurements have been carried out by means of cyclic voltammetry, quasi-steady polarization curves (low-overpotential approximation) and ac impedance. The experimental picture indicates that the kinetics of electron transfer are more complex than that suggested by a classical outer-sphere mechanism.