The stable tyrosyl radicals in E. coli ribonucleotide reductase (RNR) and photosystem II (PSII) were studied by high-field 245 GHz 8.7T EPR. The relaxation properties of Tyr-D degrees, the stable radical in PSII, are different depending on the spin-state of the manganese-cluster within the protein. Microwave power studies indicated that at high fields and low temperatures the RNR radical relaxes comparably to Tyr-D degrees when the Mn-cluster is in the paramagnetic state. This verified the existence of a weak exchange coupling between the tyrosyl radical and the nearby binuclear iron site in RNR. A detailed study of the g-values of the two radicals was carried out, and these g-values were compared to those obtained from an existing 9 GHz single-crystal EPR study of a model tyrosyl radical. The g-values of the radicals were calculated using the unrestricted Hartree-Fock MNDO molecular orbital method with PM3 parameterization and the theory of g-values of radicals developed by Stone. The spin-orbit coupling contribution to the g-values were evaluated using a modified version of Roothaan’s theory of electronic excitation. The local environment of the radicals were incorporated into these calculations by inclusion of point charges and hydrogen bonds. The method described here is independent of adjustable parameters except those inherent to the PM3-MNDO method.