Following our previous studies confirming the role of adsorbed electrocatalytic SCN- ions in the formation of negative polarisation resistance in the electroreduction of the Ni(II)SCN- complexes at the mercury electrode, we present further experimental and theoretical investigations of the oscillatory behaviour of this process. Examples of current oscillations collected for various experimental conditions are shown. A numerical model involving partial differential equations of diffusion transport is constructed for simulations of reported oscillations. Comparison of experimental and theoretical oscillations confirms both the electrochemical source of oscillations and the validity of the numerical approach proposed. With this approach it was possible to explain the effect of a side formation of the inhibitory layer of NiS on the shapes of oscillations.