Preliminary EXAFS data has been collected on the molybdennum ( K-edge) in C. pasteurianum formate dehydrogenase and the tungsten ( L III-edge) in C. thermoaceticum formate dehydrogenase. In the presence of dithionite, the tungsten enzyme was devoid of W = O bonds, and exhibited average W-(O, N) and W-S bond lengths of 2.13 ± 0.03 Å and 2.39 ± 0.03 Å, respectively. In sharp contrast, the C. pasteurianum molybdenum site has three Mo = O bonds with an average bond lengths of 1.74 ± 0.03 Å. It is also the first molybdenum enzyme found lacking Mo-S bonds, and does not appear to be redox active in the presence of formate or dithionite. Model compounds WO 2(8-hydroxyquinoline) 2 = WO 2(ox) 2, and WO 2(8 mercaptoquinoline) 2 = WO 2(tox) 2, were also examined. Respective predicted bond lengths for WO 2(ox) 2 and WO 2(tox) 2 were W = O of 1.71, 1.73 Å; W-N of 2.31,2.29 Å; W-O or W-S of 1.92 or 2.40 Å, with estimated uncertainties of ±0.03 Å.