Molybdenum is centrally involved in several vital physiological functions, including the oxidation of sulfite to sulfate in sulfite oxidase (SO) as part of amino acid catabolism.  At the heart of SO is a 'molybdenum cofactor', the defect or deficiency of which is known to result in the severe neurological symptoms of 'sulfite oxidase deficiency' in human infants (e.g. dislocated ocular lenses, mental retardation, attenuated growth of the brain, and early death).  High resolution pulsed EPR studies of mutant forms of SO will provide a more detailed understanding of the biochemical function of the enzyme and its mechanism.  In this work, we are interested in developing model systems that will provide benchmark EPR parameters for molybdenum(V)-chloro, -oxo, and –hydroxo species and allow better interpretation of the EPR of the related enzymes.  Previously studied models have been prepared either with nitrogen (I = 1) donor atoms, which strongly modulate the pulsed EPR signals from molybdenum(V) and hinder the observation of coupling from other nuclei, or with naturally abundant oxygen, which has no nuclear spin (99.9 % I = 0) and, as such, exhibits no nuclear coupling at all.  The syntheses and pulsed EPR studies of new model complexes that have chloro and 17-O labeled oxo/hydroxo ligand sets using I = 0 supporting ligands will be presented.  Where possible, the pulsed EPR data for these well characterized model compounds will also be compared with studies of the molybdenum enzymes.