The invention relates to a method for the selective catalytic reduction of nitrogen oxides using ammonia in exhaust gases of vehicles, whereby solutions of guanidine salts with an ammonia forming potental of between 40 and 850 g/kg, optionally in combination with urea and/or ammonia and/or ammonium salts, are catalytically decomposed in the presence of catalytically active, non-oxidation-active coatings of oxides selected from the group containing titanium dioxide, aluminium oxide, silicon dioxide or the mixtures thereof, and hydrothermally stable zeolites which are fully or partially metal-exchanged. The guanidine salts according to the invention enable a reduction of the nitrogen oxides by approximately 90 %. Furthermore, said guanidine salts can enable an increase in the ammonia forming potential from 0.2 kg, corresponding to prior art, up to 0.4 kg ammonia per litre of guanidine salt, along with freezing resistance (freezing point below -25°C). The risk of corrosion of the guanidine salt solutions used a
