Three decatungstates (DTs) were conveniently synthesized respectively using quaternary ammonium cations of tetramethyl, tetrapropyl and tetrabuthyl as counterions (marked as TMADT, TPADT and TBADT) and their quality, photoluminescence (PL) property and redox capacity were measured by UV-vis, XPS and PL spectra and cyclic voltammograms (CVs), respectively. The photo -catalysis activities of three DTs were checked using visible light-driven selective oxidation of cyclohexane, toluene, ethylbenzene and benzyl alcohol by dioxygens (O-2) in acetonitrile (MeCN). The results showed that the photo -catalysis activity of DT anion for these oxidations gradually increases with the shortening of its cationic alkyl chain, which is attributed to the continuous improvement of its quality from TBADT to TMADT. Furthermore, the additives water, 12 M hydrogen chloride (HCl) solution and especially the mixture of both can markedly promote the DTs-photocatalyzed oxidations and such promotion presents an enhancing trend from TBADT to TMADT. which is likely due to the beneficial effects of these additives as follows: i) Water can significantly stabilize the structure of DT anion under light illumination and improve its redox cycling in MeCN; ii) 12 M HCl may obviously improve the oxidative capacity and redox cycling of DT anion and stabilize its photo-excited states. iii) The respective promotion effects of such two additives on photo -catalysis can be well combined and such combined effect is enhanced drastically upon TMADT owing to its hydrophilic feature, thus significantly improving the photo-catalytic efficiency of TMADT under the joint action of water and 12 M HCl.