Sulfite oxidizing enzymes (SOEs) are physiologically vital and occur in all forms of life. During the catalytic cycle the five-coordinate square-pyramidal oxo-molybdenum active site passes through the Mo(V) state, and intimate details of the structure can be obtained from pulsed EPR spectroscopy through the hyperfine interactions (hfi) and nuclear quadrupole interactions (nqi) of nearby magnetic nuclei (e.g. ¹H, ²H, ¹⁷O, ³¹P) of the ligands. By employing spectrometer operational frequencies ranging from ~4 to ~32 GHz, it is possible to make the nuclear Zeeman interaction significantly greater than the hfi and nqi, and thereby simplify the interpretations of the spectra. One type of structure has a single exchangeable Mo-OH proton approximately in the equatorial plane and a large isotropic hfi; a second type has two exchangeable protons with distributed orientations out of the equatorial plane and very small (or zero) isotropic hfi; a third type has no nearby exchangeable protons and a coordinated oxyanion. The particular Mo(V) structure observed depends upon the organism, pH, anions in the media, mode of reduction, and point mutations. The ¹⁷O (I = 5/2) hfi and nqi parameters for SOEs in solutions enriched in H₂¹⁷O showed the unexpected result that the axial oxo group exchanges with ¹⁷O. The first determination of oxo ¹⁷O nqi parameters for a well-characterized model compound, [Mo¹⁷O(SPh)₄]^-, clearly demonstrated that ¹⁷O nqi parameters can distinguish between oxo and OH₂ ligands.