The Cp(2)Fe(+) oxidation and the protonation of CpMoH(CO)(2)L (L : PPh(3), 1; PMe(3), 2) in MeCN have been investigated. Tn the dry solvent, the oxidation of both compounds consumes 1 mol of oxidant/mol of hydride with production of [CpMo(CO)(2)L(MeCN)](+) (L : PPh(3), [3](+); PMe(3), [4](+)) and H-2. The stoichiometry changes toward the consumption of 2 mol of oxidant in the presence of excess water when the oxidizing equivalents are added rapidly, either chemically or electrochemically. However, 1 oxidizing equiv is again sufficient to consume the hydride material completely under conditions of slow oxidation. Under comparable conditions, the more basic 2 leads to a lower [ox]/M-H stoichiometry. Protonation of 1 and 2 with HBF4 . Et(2)O in dry MeCN results in rapid H-2 evolution and formation of [3](+) and [4](+), respectively, whereas the presence of excess water suppresses the H-2 evolution and gives rise to protonated water. However, this process is followed by slow and irreversible delivery of the proton back to 1 or 2 to afford [CpMoH(2)(CO)(2)L](+), which ultimately decomposes to [3](+) or [4](+) and H-2. The dihydride complex is too unstable to be isolated, even when the protonation of 1 or 2 is carried out in a noncoordinating solvent. The proton delivery is faster for the more basic 2 and slower for the less basic 1. Thus, water operates as a "proton shuttle", whose speed depends on the basicity difference between the hydride complex and water. The identity of the protonated water in MeCN as [H(H2O)(3)](+) is suggested by an independent H-1-NMR experiment in CD3CN.