Method for detecting and quantifying leaks of ammonia downstream of a system for the selective catalytic reduction of nitrogen oxides which is placed in an exhaust line of a motor vehicle internal combustion engine, said system comprising: an injection device for injecting an ammonia-containing fluid into the exhaust line; and a catalytic-reduction brick, downstream of the injection device, for storing the ammonia derived from the injection device with a view to it reacting with the nitrogen oxides contained in the combustion gases from the engine; said method comprising steps consisting in: (a) generating a control signal for controlling the quantity of ammonia-containing fluid injected by the injection device into the exhaust line; (b) receiving a sequence of measured-concentration values derived from a nitrogen oxide sensor positioned downstream of the system; (c) for each value received in step (b), receiving a value for the concentration of nitrogen oxides upstream of the system; (d) calculating a sequence of values indicative of the measured efficiency of the catalytic-reduction brick as a function, in the case of each of said calculated values, of a value received in step (b) and of the corresponding value received in step (c); (e) evaluating (A5, B5, 64) a reference-efficiency value ( η slip) for the catalytic-reduction brick, from at least one value of the sequence calculated in step (d), corresponding to a quantity of injected fluid that is such that the catalytic-reduction brick stores enough ammonia for the nitrogen oxides reduction reaction produced to be a total reduction and whereas, at the same time, the catalytic-reduction brick has the ability to store unsaturated ammonia; and (f) quantifying the ammonia concentration in the exhaust gases downstream of the selective catalytic reduction system as a function of: - the reference-efficiency value ( η slip) for the catalytic-reduction brick; - at least one measured-concentration value derived from the nitrogen oxides sensor positioned downstream of the system; and - the corresponding value, in the case of said at least one measured-concentration value, of nitrogen oxide concentration upstream of the system. an injection device for injecting an ammonia-containing fluid into the exhaust line; and a catalytic-reduction brick, downstream of the injection device, for storing the ammonia derived from the injection device with a view to it reacting with the nitrogen oxides contained in the combustion gases from the engine; (a) generating a control signal for controlling the quantity of ammonia-containing fluid injected by the injection device into the exhaust line; (b) receiving a sequence of measured-concentration values derived from a nitrogen oxide sensor positioned downstream of the system; (c) for each value received in step (b), receiving a value for the concentration of nitrogen oxides upstream of the system; (d) calculating a sequence of values indicative of the measured efficiency of the catalytic-reduction brick as a function, in the case of each of said calculated values, of a value received in step (b) and of the corresponding value received in step (c); (e) evaluating (A5, B5, 64) a reference-efficiency value ( η slip) for the catalytic-reduction brick, from at least one value of the sequence calculated in step (d), corresponding to a quantity of injected fluid that is such that the catalytic-reduction brick stores enough ammonia for the nitrogen oxides reduction reaction produced to be a total reduction and whereas, at the same time, the catalytic-reduction brick has the ability to store unsaturated ammonia; and (f) quantifying the ammonia concentration in the exhaust gases downstream of the selective catalytic reduction system as a function of: - the reference-efficiency value ( η slip) for the catalytic-reduction brick; - at least one measured-concentration value derived from the nitrogen oxides sensor positioned downstream of the system; and - the corresponding value, in the case of said at least one measured-concentration value, of nitrogen oxide concentration upstream of the system. - the reference-efficiency value ( η slip) for the catalytic-reduction brick; - at least one measured-concentration value derived from the nitrogen oxides sensor positioned downstream of the system; and - the corresponding value, in the case of said at least one measured-concentration value, of nitrogen oxide concentration upstream of the system.