Disclosed is a process for the dynamic monitoring of the flow rate of liquid additive consumed by a liquid-additive injector of an exhaust gas treatment system of a motor vehicle. The measurement of the pressure of the liquid makes it possible firstly to deduce the flow rate circulating through the orifice and secondly, by knowing the operating characteristic of the pump, to determine the flow rate of liquid additive actually delivered to the system for treating polluting gases. The process also provides a phase of characterizing the pump, including commanding the closure of the injector, measuring at least two pressure values for two different operating speeds of the pump, and updating the pump operating characteristics table on the basis of the pressure values measured.