Abstract: An exhaust gas aftertreatment device for internal combustion engines having a catalyzer for the selective catalytic reduction of oxides of nitrogen from exhaust gases of motor vehicle diesel engines, provides overstoichiometric supply of NH.sub.3 or materials releasing NH.sub.3. A first sensor records the NH.sub.3 concentration contained in the exhaust gas and interrupts the supply of the NH.sub.3 quantity when a specified upper threshold value is reached. A second sensor records the NH.sub.3 adsorbed in the catalyzer, by way of which the NH.sub.3 supply is resumed on reaching a specified lower threshold value. Alternatively, only one NH.sub.3 sensor is provided in the exhaust gas aftertreatment device. The NH.sub.3 concentration determined by this single sensor is compared, as the actual value, with a required value corresponding to a specified NH.sub.3 concentration in order to form a correction signal which is used for triggering the metering appliance continuously connected into the gas phase.