The molybdenum(V) coordination environment of sulfite oxidase has been investigated by multifrequency ESEEM spectroscopy in similar to 70 mM phosphate buffer at pH* = 6.5 in both H2O and D2O. The FT-ESEEM spectra in H2O typically consist of three lines. One of these lines is always close to twice the Larmor frequency of the P atom (2 nu(P)) and is assigned to one or more coordinated phosphates, providing the first direct unambiguous detection of such coordination. Extensive simulations of this phosphate signal at the various operational frequencies indicated that the coordinated phosphate group(s) probably does (do) not adopt a fixed orientation, and as a result, a description of the Mo...P hyperfine interaction required the introduction of a distribution of such orientations, with Mo...P distance(s) of 3.2-3.3 Angstrom. The other two lines in the FT-ESEEM spectra in H2O, located at nu(H) and 2v(H), were assigned to matrix protons, In D2O buffer two additional lines, assigned to matrix deuterons, were also seen.