During the etching of AZ 1350 photoresist in O2 and O2/CF4 discharges, ground-state concentrations of atoms (O, F, and H), and small radicals (OH, HO2, RO2) were measured in the discharge afterglow by EPR spectroscopy. In the case of CF4/O2 discharges, the dependence of O and F atom concentrations on the etch time reflects both surface oxidation and fluorination reactions in accordance with existing etch models. In the case of high-rate resist etching in pure O2 discharges, high concentrations of product radicals (H, OH and HO2) were detected and compared with resist free O2/H2O discharges. Kinetic modeling of the afterglow reactions reveals that the mean lifetime and, accordingly, the diffusion length of the etchant species O(3P) is drastically reduced in rapid reactions with OH and HO2. The results are used to simulate both etch homogeneity and the loading effect in a simple etch model.