The kinetics of O and F atoms in O2‐based plasmas has been studied by time‐resolved optical emission spectroscopy (actinometry) in modulated plasmas. The sticking coefficient αO of O atoms on the stainless‐steel reactor walls was 0.09±0.01 in O2 plasmas containing fluorine (added as either SF6 or F2), but was about 0.5 in a pure O2 plasma. This explains the significant increase in steady‐state O density as a few percent of fluorinated gas is added. The corresponding value for F atoms, αF, was 0.06±0.01, almost independent of conditions. The method also indicates the relative importance of the different electron‐impact‐induced mechanisms (direct excitation of ground‐state atoms and dissociative excitation of feedstock molecules) for the production of emitting atoms [O 3p3P (844 nm) and F 3s2P (703 nm)] in plasmas. These results show that the widely used (steady‐state) actinometry technique using 844‐nm emission from O 3p3P atoms is an unreliable measure of ground‐state [O] variations.