A method of estimating the quantity of secondary reaction airflow supplied to the exhaust manifold of an internal combustion engine by an air injection reaction (AIR) system models airflows in an AIR pump, an AIR valve, and a plenum coupling the pump to the valve. The pump flow is modeled based on the pump speed (or pump motor voltage), the pump inlet air temperature and the pressure ratio across the pump, and the valve flow is modeled based on the pressure ratio across the valve and the valve inlet air temperature. The plenum pressure used in the pump and valve pressure ratios is modeled based on the net plenum airflow and the plenum air temperature. The estimated AIR airflow is useful for managing the air/fuel ratio in the engine exhaust system and enhancing the reliability of AIR diagnostics.