The heteropoly anion clusters [S2Mo18O62](4-) and [S2Mo18O62](5-) may be photochemically activated to generate extremely powerful oxidants. Quantitative photoelectrochemical measurements using laser excitation at 325 nn of acetonitrile solutions of [S2Mo18O62](4-) containing the electron donors (ED), toluene and tetrahydrofuran, with hydrodynamic voltammetric monitoring at a platinum channel electrode are consistent with the following CECE mechanism : [S2Mo18O62](4-) + ED (k(1)(hv)) --> [S2Mo18O62](5-) + ED(+) (C); [S(2)MO(18)O(62)](5-) reversible arrow [S2Mo18O62]4- + e (E); [S2Mo18O62](5-) + ED (k(2)(hY)) --> [S2Mo18O62](6-) + ED(+) (C); [S2Mo18O62](6-) reversible arrow [S2Mo18O62](4-) + 2e(-) (E); ED(+) (fast) - products. Photochemical oxidation of other electron donors such as methanol, [BPh(4)](-), ferrocene, and Cr(CO)(3) (arene) also occurs to generate [S2Mo18O62](5-) and [S2Mo18O62](6-) but not the more highly reduced [S2Mo18O62](7-) and [S2Mo18O62](8-) anions. It is concluded on the basis of these studies that [S2Mo18O62](4-) is a potentially valuable photooxidant in the synthetic sense, since it may be readily synthesised in large quantities from inexpensive materials.