Sulfite oxidase from Arabidopsis thaliana has been reduced at pH = 6 with sulfite labeled with S-33 (nuclear spin I = 3/2), followed by reoxidation by ferricyanide to generate the Mo(V) state of the active center. To obtain information about the hyperfine interaction (hfi) of S-33 with Mo(V), continuous-wave electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) experiments have been performed. The interpretation of the EPR and ESEEM spectra was facilitated by a theoretical analysis of the nuclear transition frequencies expected for the situation of the nuclear quadrupole interaction being much stronger than the Zeeman and hyperfine interactions. The isotropic hfi constant of S-33 determined in these experiments was about 3 MHz, which demonstrates the presence of coordinated sulfate in the sulfitereduced low-pH form of the plant enzyme.