To measure the mass flow rate of a constituent of a gaseous stream, for example, a pollutant in an engine exhaust gas, the concentration or mole fraction of the constituent is measured by an analyzer. Then a tracer gas is injected into the gaseous stream at a predetermined rate and mixed therewith and the concentration of the tracer gas is measured. Then the mass flow rate of the constituent is calculated by the equation Wc Cc Mc Wc /Ct Mt where Wc and Wt are the mass flow rates of the constituent and the tracer gas respectively, Cc and Ct are the concentrations of the constituents and the tracer gas respectively, and Mc and Mt are the molecular weights of the constituent and the tracer gas respectively. Several constituents may be monitored simultaneously in this manner by measuring the concentration of each constituent and using only the one measurement for the tracer gas.