No colour change occurred when a colourless, aqueous solution of the β-octamolybdate(VI) salt [Et3NH]4[Mo8O26] (1) was irradiated with UV light (λ = 254 nm). Similarly, an irradiated solution of 1 in a water: (±)-2-butanol mixture also remained colourless and no gaseous hydrogen evolved. In contrast, aqueous solutions of the alkali and alkaline earth metal containing salts [Et3NH]3[NaMo8O26] (2) and [Et3NH]2[Mg(H2O)6Mo8O26]·2H2O (3) both turned blue (γmax= 725 nm) when irradiated with UV light. Irradiated solutions of 2 and 3 in a water: (±)-2-butanol mixture also turned blue, and 2-butanone along with hydrogen gas were produced in almost quantitative yield over a 24 h reaction period. Likewise, an aqueous solution containing 1 and excess of an alkali or alkaline earth metal chloride salt (e.g. NaCl, KCl, MgCl2, CaCl2 or SrCl2) also catalysed the photooxidation of (±)-2-butanol. Product yields were again ⩾ 90%.