Degradation and deactivation mechanisms of an Sn catalyst used for CO2 reduction to formate in 2 M KCl solution was investigated at various potentials by using a rotating disk electrode. Surface degradation was studied by SEM/EDS and chemical analysis was performed by XPS. The results indicated that the optimum potential for CO2 reduction on Sn electrode was similar to -1.8 V-SCE where maximum faradaic efficiency and minimum degradation were obtained. Two types of degradation were observed after cathodic polarization: cathodic corrosion resulting in the potential loss of catalyst and formation of alkali metal deposits resulting in deactivation of the catalyst. The former type gave a crystallographic type of etching morphology near and at the grain boundaries after polarization in the potential range of -1.8 to -2.2 V-SCE. The number of attacked areas increased at more negative potentials and with increasing electrode rotation rate. Corrosion did not cause electrode deactivation as indicated by a relatively stable faradaic efficiency. Moreover, the material loss was insignificant because only small areas were affected. Corrosion was attributed to the formation of tin hydride. Meanwhile, formation of intermetallic compound (KSn), which occurred during simultaneous CO2 reduction reaction at all potentials tested in this work, led to both cathodic deactivation and material loss. The existence of KSn deposits on the electrode surface was detected by EDS. The formation of intermetallic compound was always accompanied by an increase in current output possibly due to hydrogen evolution. Heavy colloidal deposits of KSn were found. For use of an Sn catalyst in a device, it is necessary to develop a technique to avoid or remove the alkali metal deposit. (C) 2014 Elsevier Ltd. All rights reserved.