The present invention concerns a process for selectively removing nitrogen oxides (NOx) from exhaust or flue gasses, especially from gases arising from the burning of biomass, combined biomass-fossil fuel and from waste incineration units, and a catalyst for this process. The process relates to the selective catalytic reduction (SCR) of NOx with ammonia as reductant which is the most common method to catalytically reduce NOx in flue gasses from stationary sources. SCR systems are designed to clean the nitrogen dioxide (NO 2 and nitrogen oxide (NO) out of flue gasses. The present invention relates to a process for the selective removal of nitrogen oxides in the presence of ammonia from gases containing a significant amount of alkali metal and/or alkali-earth compounds which process comprises using a catalyst combined of a) a formed porous superacidic support b) a metal oxide catalytic component deposited on said support selected from the group consisting of Cu, V, Fe, Cr, Mn, Mo and mixtures thereof. The support can e.g. be obtained by the adsorbing of sulfate ions onto amorphous or crystalline inorganic oxide of Zr, Ti, Fe, Sn, Si, Al, and Hf, followed by calcination in air.