A method and a device for providing ammonia for the selective catalytic reduction of nitrogen oxides in the exhaust system of an internal combustion engine, include the following steps: a) recording at least one actuating variable for influencing the state of the internal combustion engine; b) calculating a possible nitrogen oxide emission of the internal combustion engine on the basis of an evaluation of the at least one measured variable; c) calculating an ammonia quantity which corresponds to the possible nitrogen oxide emission; and d) initiating the provision of the corresponding ammonia quantity by exhaust-gas-external evaporation in an evaporator and exhaust-gas-external hydrolysis of aqueous urea solution, wherein at least one of the following values: i) the heating power of the evaporator or ii) the quantity of aqueous urea solution supplied to the evaporator, is regulated on the basis of the ammonia quantity calculated in step c).