The invention relates to a method for the selective catalytic reduction o f nitrogen oxides using ammonia in exhaust gases of vehicles, whereby soluti ons of guanidine salts with an ammonia forming potental of between 40 and 85 0 g/kg, optionally in combination with urea and/or ammonia and/or ammonium s alts, are catalytically decomposed in the presence of catalytically active, non-oxidation-active coatings of oxides selected from the group containing t itanium dioxide, aluminium oxide, silicon dioxide or the mixtures thereof, a nd hydrothermally stable zeolites which are fully or partially metal-exchang ed. The guanidine salts according to the invention enable a reduction of the nitrogen oxides by approximately 90 %. Furthermore, said guanidine salts ca n enable an increase in the ammonia forming potential from 0.2 kg, correspon ding to prior art, up to 0.4 kg ammonia per litre of guanidine salt, along w ith freezing resistance (freezing point below -25°C). The risk of corrosion of the guanidine salt soluti
