Abstract: A method of measuring the quantity of an ingredient gas in the exhaust gas from a motor vehicle during each of a sequence of driving modes by measuring and storing in real time flow rate data Q.sub.E (t) representative of the flow rate of the exhaust gas at a succession of times during a driving mode, measuring and storing concentration data representative of the concentration of the ingredient gas in the exhaust gas during a time period delayed with respect to the time period of the driving mode in order to compensate for a time delay in correspondence between changes in flow rate and performing an interpolation method and either a data-compression or data-expansion method on the concentration data in order to obtain corrected concentration values C.sub.E (t) on a one-to-one basis in phase and magnitude to the flow rate data, and determining the quantity M(t) of the ingredient gas in the exhaust gas for each of the measurements of flow rate based on the operational equation M(t)=.rho..times.C.sub.E (t).