In this work, we studied the performance of Co catalysts supported on different carbonaceous structures (activated carbon, carbon nanofibers and carbon nanotubes) for the steam reforming of ethanol at 773 K. Co supported on activated carbon and carbon nanofiber exhibited the smallest Co particle size and the highest activity. All catalysts deactivated but the loss of activity depended on the carbon structure. Co/C and Co/CNT catalysts significantly deactivated, while only a slight decrease in activity was observed for Co/CNF. TG analysis did not detect the formation of carbon deposits. In situ XANES and EXAFS showed that Co was partially oxidized after reduction at 623 K and the catalyst was further reduced during reaction, which rules out the oxidation of the metallic particles as the reason for catalyst deactivation. TEM of the fresh and spent catalyst showed a significant growth of the particles for Co/C and Co/CNT, indicating that sintering is the main cause of catalyst deactivation. Co/CNF only slightly deactivated because Co sintering extent was low.