Commercial vanadia-based SCR monoliths have been exposed to flue gases in a pilot-scale Setup into which phosphoric acid has been added and the deactivation has been followed during the exposure time. Separate measurements by SMPS showed that the phosphoric acid formed polyphosphoric acid aerosols, which were characterized by particle number concentrations in the order of 1 x 10(14) #m(3) at 350 degrees C and diameters <0.1 mu m. Three full-length monoliths have been exposed to flue gases doped with 10, 100 and 1000 ppmv H3PO4 for 819, 38 and 24 h. respectively. At the end of the exposure the relative activities were equal to 65, 42 and 0%, respectively. After exposure, samples of the spent monoliths have been characterized by ICP-OES. Hg-porosimetry. SEM-EDX and in situ EPR. The results showed that the polyphosphoric acids chemically deactivate the vanadia-based catalysts by decreasing the redox properties of the catalyst surface and by titrating the number of V(V) active species. When plate-shaped commercial catalysts have been wet impregnated with different aqueous solutions of H3PO4 obtaining P/V ratios in the range 1.5-5, the relative activity for the doped catalysts in the whole P/V range was 0.85-0.90 at 350 degrees C. These results show that the presence of phosphor compounds in the flue gas may be much more harmful than indicated by simple wet chemical impregnation by phosphoric acid. The reason has been found in the nature of the polyphosphoric acid aerosol formed in the combustion process, which cannot be reproduced by the wet impregnation process. (C) 2008 Elsevier B.V. All rights reserved.