Different functions of the carbonaceous deposits have been recognised in ethylene hydrogenation (150degreesC < T < 450degreesC) over the model cobalt foil catalyst: retarding, blocking and activating. The carbonaceous deposits were investigated by TPO, TPR and SEM methods. Their quantitative results were related to the number of metallic active centres measured also by TPO and TPR. It has been shown that retarding the reaction rate by the hydrocarbon deposit takes place below 300 degreesC; blocking the catalyst by the dehydrogenated deposit occurs between 300 and 400 degreesC; restoring the catalyst activity above 400 degreesC. The renewed activity was explained by the diffusion of cobalt atoms to a deposit surface.