Method for determining a quality of a reducing agent solution containing ammonia, which is used in an SCR exhaust gas purification system of an internal combustion engine to reduce nitrous oxides, in which method a metering unit is activated for delivering presettable target quantities of the reducing agent solution into the internal combustion engine exhaust gas and, by means of a NOx sensor with a cross-sensitivity to ammonia, which is located downstream of an SCR catalytic converter of the SCR exhaust gas purification system, an efficiency variable which at least correlates to the efficiency (ε) of the SCR catalytic converter is determined and compared to a presettable limit value, wherein - in a first step of the method, a first target metering quantity and a first limit value are provided in a normal metering mode, - if an efficiency variable which corresponds to an improper function of the SCR exhaust gas purification system is detected by the comparison, the normal metering mode is aborted and the system switches in a second step of the method to a diagnostic mode, in which a second target metering quantity higher than the first target metering quantity and a second limit value are provided for a first time interval, - if a presettable deviation of the efficiency variable from the second limit value is detected on expiry of the first time interval, the SCR catalytic converter is, in a third step of the method, conditioned in such a way that an ammonia quantity stored in the SCR catalytic converter falls below a presettable storage quantity limit value, and - in an adaptation mode serving a the fourth step of the method, a third target metering quantity and a third limit value are provided for a presettable second time interval, wherein, if a presettable deviation of the efficiency variable from the third limit value is detected on expiry of the second time interval, a faulty quality of the reducing agent solution is diagnosed. - in a first step of the method, a first target metering quantity and a first limit value are provided in a normal metering mode, - if an efficiency variable which corresponds to an improper function of the SCR exhaust gas purification system is detected by the comparison, the normal metering mode is aborted and the system switches in a second step of the method to a diagnostic mode, in which a second target metering quantity higher than the first target metering quantity and a second limit value are provided for a first time interval, - if a presettable deviation of the efficiency variable from the second limit value is detected on expiry of the first time interval, the SCR catalytic converter is, in a third step of the method, conditioned in such a way that an ammonia quantity stored in the SCR catalytic converter falls below a presettable storage quantity limit value, and - in an adaptation mode serving a the fourth step of the method, a third target metering quantity and a third limit value are provided for a presettable second time interval, wherein, if a presettable deviation of the efficiency variable from the third limit value is detected on expiry of the second time interval, a faulty quality of the reducing agent solution is diagnosed.