The activity, long-term stability and the reasons for deactivation of a nanosize gold-supported Au/TiO2 in CO oxidation were investigated. Characterization of the catalyst sample was made by XPS, FTIR, TEM, BET, "depletive" oxidation and TPD instrumental methods. In spite of the experimentally proved very high activity at temperatures below 213 K, the catalyst exhibited a gradual decrease in initial activity. Two main reasons for the catalyst deactivation were found: (i) capability to adsorb CO and accumulate it as carbonates, this deactivation is reversible and after heating the catalyst surface is restored by CO2 evolution; (ii) agglomeration of Au particles, which causes irreversible, however weak deactivation. (C) 2003 Elsevier B.V. All rights reserved.