A system and method for real time deposition process control based on resulting product detection, where the system and method detect an amount of at least one reaction product in real time, while the deposition process is being performed, the detected amount of reaction product is compared with a reference amount, and a comparison result is fed back in real time to adjust a supply of one or more reactants. The system and method provide real time control over the deposition process and/or reduce the number of wafers produced that do not meet processing target values.