The study of an electrocatalytic mechanism is presented, Closed form solutions are given for the steady-state behaviour, facilitating the linear stability analysis. It is shown that two types of oscillations may be exhibited for this self-oscillating electrocatalytic reaction : small amplitude harmonic oscillations and large amplitude relaxation oscillations. The existence of these oscillations and their form depend on the reaction conditions. The Hopf bifurcation is supercritical and small amplitude harmonic oscillations exist only over a very narrow range of electrode potentials, The electrocatalytic reaction behaviour is that of a so-called ＇canard＇.