We report a multifrequency (9.6-, 94-, 190-, and 285-GHz) EPR study of a freeze-quenched intermediate obtained from reaction of substrate-free cytochrome P450cam (CYP101) and its Y96F and Y96F/Y75F mutants with peroxy acids. It is generally assumed that in such a shunt reaction an intermediate [Fe(IV)=O, porphyrin-pi-cation radical] is formed, which should be identical to the species in the natural reaction cycle. However, for the wild type as well as for the mutant proteins, a porphyrin-pi-cation radical is not detectable within 8 ms. Instead, EPR signals corresponding to tyrosine radicals are obtained for the wild type and the Y96F mutant. Replacement of both Tyr-96 and Tyr-75 by phenylalanine leads to the disappearance of the tyrosine EPR signals. EPR studies at 285 GHz on freeze-quenched wild type and Y96F samples reveal g tensor components for the radical (stretched g(x) values from 2.0078 to 2.0064, g(y) = 2.0043, and g(z) = 2.0022), which are fingerprints for tyrosine radicals in a heterogeneous polar environment. The measurements at 94 GHz using a fundamental mode microwave resonator setup confirm the 285-GHz study. From the simulation of the hyperfine structure in the 94-GHz EPR spectra the signals have been assigned to Tyr-96 in the wild type and to Tyr-75 in the Y96F mutant. We suggest that a transiently formed Fe(IV)=O porphyrin-pi-cation radical intermediate in P450cam is reduced by intramolecular electron transfer from these tyrosines within 8 ms.