Mo(VI)(S(2)C(6)H(4))(3) reacts cleanly and completely with H(2)O in THF to afford [H(3)O](+)[Mo(V)(S(2)C(6)H(4))(3)](-). Kinetic data were fit by the rate equation -d[Mo(VI)(S(2)C(6)H(4))(3)]/dt = k[Mo(VI)(S(2)C(6)H(4))(3)]/[H(3)O(+)], which is consistent with a coupled electron-proton transfer mechanism involving a coordinated H(2)O molecule. The Mo(VI)(S(2)C(6)H(4))(3) reduction is accelerated by the presence of PPh(3) and affords OPPh(3). (18)O isotope tracing shows that H(2)O is the source of oxygen transferred to PPh(3).