A method of regulating the amount of reducing agent added, especially NH.sub.3 in the case of the catalytic reduction of NO.sub.x of flue gases which originate from a combustion installation which is fired with fossil fuels. The adjustment value for the quantity to be added is determined in response to a prescribed reducing agent/NO.sub.x stoichiometry factor from the quantity of combustion air supplied to the combustion installation or from the exiting quantity of flue gas, and from the NO.sub.x concentration downstream ahead of the catalyzer. Regulation is effected by influencing the stoichiometry factor. The NO.sub.x concentration downstream after the catalyzer is returned to the regulation process as the primary correction value, and the reducing agent concentration downstream after the catalyzer, and the flue gas temperature ahead of the catalyzer, are returned to the regulation process as the secondary correction values. The set stoichiometry factor is influenced such that while maintaining the emission limit value as the desired value of the primary correction value, and while maintaining a reducing agent concentration downstream after the catalyzer below a prescribed fixed value in a range of from 50 to 3 ppm volume, preferably less than 10 ppm volume, the stoichiometry factor is regulated via the temperature-dependent activity curve of the catalyzer.