It is challenging to obtain absolute rate constants for the quenching of organic radicals by molecular oxygen because they often do not present absorbance in the UV-vis range. Here, it is shown that time-resolved EPR (chemically induced dynamic electron polarization, or CIDEP) spectroscopy is useful in establishing rate constants for the addition of benzoyl radicals to molecular oxygen. It was found that benzoyl radicals are particularly reactive toward O-2 and can, therefore, act as oxygen scavengers in the initiating phase of radical polymerizations. Kinetic simulations underpin this reactivity.