Four states of the cobalt foil catalyst, corresponding to different redox treatment and activity, were defined: oxidised, reduced, active and deactivated. They were investigated by scanning electron microscopy (SEM), temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), X-ray photoelectron spectroscopy (XPS) and thermogravimetric (TG) methods and in the hydrogenation of ethylene used as a test reaction. Particular emphasis was laid on the study of the active state, achieved after the catalyst reduction at moderate temperatures. It was shown that the catalyst preactivated by a series of redox cycles is built of a cobalt oxide layer of a characteristic size and dispersion, which is stuck to the metallic bulk. Reduction at a moderate temperature, prolonged even to several hours, converts only a small fraction of the oxide layer into metallic Co. XPS, TPR and TPO methods distinguished various states of oxygen and cobalt on the surface of the activated or partially activated samples. The results were interpreted in terms of the mechanism of autocatalytic reduction. The deactivation was associated with the structural reconstruction of the surface, taking place either in the reaction mixture during the hydrogenation of ethylene or in hydrogen atmosphere. Formation of the inactive carbon deposit was experimentally excluded.