Decay mechanism of the V2O5/TiO2 commercial catalyst in the coal combustion flue gas was studied. Both fresh and spent catalysts with honeycomb structure, which were exposed to a coal combustion flue gas in a coal-fired boiler for over 65,000 h. were examined. The catalysts were characterized by BET analysis, FE-SEM observation with EDX analysis, and catalytic activity of Hg-0 oxidation and NO reduction was measured. Fly ash particles were observed to deposit on the spent catalyst and form the layer-like porous structure. The thickness of the ash deposition layer was 3.1 mu m at the front-end of the catalyst and observed to have decreased to 0.9 mu m at the back-end. The activity loss was observed to be more pronounced at the front-end of the catalyst where the most severe ash accumulation was confirmed than at the back-end of the catalyst for both Hg-0 oxidation and NO reduction. The catalytic activity, both Hg-0 oxidation and NO reduction, was found to decrease proportionally with the ash deposition thickness. The diffusion of the reactant through the porous ash deposition layer is the prevailing rate limiting step which is the main factor of the deactivation. (C) 2012 Elsevier B.V. All rights reserved.