Different deactivation modes in ethylene hydrogenation have been studied on the model cobalt catalyst by TPR/O and SEM methods. The focus in this study was on the low temperature deactivation of the catalyst, which was found to be caused by hydrogen enhanced surface reconstruction. At higher temperatures formation of carbon deposit was confirmed as the main deactivation reason. The above methods allowed to distinguish various kinds of carbonaceous deposits and relate their amounts to the no. of active centers.