The polyoxometalate, alpha-[S2Mo18O62](4-) is reduced by phosphines in the mixed-solvent medium CH3CN/H2O (95/5 v/v). Monitoring of reaction pathways by voltammetric and NMR (O-17, P-31) techniques shows that the identity of the final products is determined by the basicity of the phosphine. Thus reaction with aryl- (Ph3P, Ph2PCH2PPh2, and Ph2PCH2CH2PPh2) and alkyl- (Et3P and (Bu3P)-Bu-n) phosphines leads to two- and one-electron reduced polyoxometalate products, respectively, for example : [S2Mo18O62](4-) + Ph3P + H2O --> [HS2Mo18O62](5-) + Ph3PO + H+; 2[S2Mo18O62](4-) + 3(n)Bu(3)P + H2O --> 2[S2Mo18O62](5-) + (Bu3PO)-Bu-n + 2(n)Bu(3)PH(+). For each reaction. the primary electron-transfer step is believed to be [S2Mo18O62](4-) + R3P --> [S2Mo18O62](5-) + R3P.+ with R3P.+ then reacting with water to generate R3PO and protons. If R3P is relatively basic (R = Bu-n, Et), a 1:2 mixture of R3PO and R3PH+ is formed due to the protonation of R3P. However, if R3P (R = Ph) is a weak base, H+ preferentially initiates the following disproportionation reaction : 2[S2Mo18O62](5-) + H+ reversible arrow [HS2Mo18O62](5-) + [S2Mo18O62](4-). Reactions in the presence of light lead to significant photocatalysis. A quantitative photoelectrochemical channel electrode study demonstrates that oxidation of Ph3P by [S2Mo18O62](4-) is accelerated substantially by irradiation in the 300-400 nm wavelength range, where absorption bands of both [S2Mo18O62](4-) and [S2Mo18O62](5-) are present.