The low-temperature behavior of the selective catalytic reduction (SCR) process with feed gases containing both NO and NO2 was investigated. The two main reactions are 4NH(3) + 2NO + 2NO(2) --> 4N(2) + 6H(2)O and 2NH(3) + 2NO(2) --> NH4NO3 + N-2 + H2O. The "fast SCR reaction" exhibits a reaction rate at least 10 times higher than that of the well-known standard SCR reaction with pure NO and dominates at temperatures above 200 degreesC. At lower temperatures, the "ammonium nitrate route" becomes increasingly important. Under extreme conditions, e.g., a powder catalyst at T approximate to 140 degreesC, the ammonium nitrate route may be responsible for the whole NOx conversion observed. This reaction leads to the formation of ammonium nitrate within the pores of the catalyst and a temporary deactivation. For a typical monolithic sample, the lower threshold temperature at which no degradation of catalyst activity with time is observed is around 180 degreesC. The ammonium nitrate route is interesting from a standpoint of general DeNO(x) mechanisms: This reaction combines the features typical to the SCR catalyst with the features of the NOx storage-reduction catalyst, i.e., NOx adsorption to a basic site.