A method is provided for the selective catalytic reduction of nitrogen oxides in the exhaust gas of internal-combustion engines in motor vehicles. An electronic control device, by way of a signal of a sensor in a reducing agent tank, detects whether a reducing agent is in a frozen state. The electronic control device applies heating current to a PTC heating element in the reducing agent tank if the reducing agent is in a frozen state, calculates the resistance of the PTC heating element from the onboard voltage of the motor vehicle and the heating current, and compares the resistance with a first defined resistance threshold and/or with a first defined gradient threshold of the increase in resistance. The amount of reducing agent fed into the exhaust gas from the reducing agent tank is reduced if the first resistance threshold and/or the first gradient threshold is exceeded.