The formation of photoactive species in the Keggin-type XPMo12-nO40 polyoxometalates (POMs, n = 1-3, X =H+, tetramethyl (TMA). tetrabutyl (TBA) or cetyltrimethyl (CTMA) ammonium cation (Q(+)))-HCl system was researched in details by UV-vis and its photo-catalysis performance evaluated via the oxidation of cyclohexane by dioxygens (O-2) in acetonitrile (MeCN) under visible light irradiation The results showed that the V-V-O-M (M for V-V or Mo-IV) sites of POM can capture HC1to form a protonated photoactive species (PA, POM-((VOHM)-O-V)Cl-+(-)) and the solvent MeCN likely participates the formation of such PA species via weak coordination. This PA species, with a characteristic absorption band of 475 nm, a V-51-NMR chemical shift of -550 ppm and an oxidative potential of higher than 0.9 V, should be responsible for the present photo-catalysis oxidation. The formation of PA species was accelerated significantly with increasing the V atoms incorporated into POMs, but hampered seriously in the presence of slight excess water due to the replaced effect of water for the coordinated HC1 and MeCN. Notably, such impediment effect of water was weakened obviously over the Q(+)-containing POMs due to a hydrophobicity of these POM's. As a result, most of the Q(+)-containing POMs showed a higher activity for this photo-catalysis reaction in MeCN-HCl-H2O media than the H+-containing counterparts. Isotope tracing test of water-containing heavy oxygen O-18 (97%) in photo-catalysis reaction indicated that the oxygen atoms in water had partly contribution to the formation of cyclohexanol (ca.8.2%) and especially cyclohexanone (ca. 33.3%), which can drastically restrain chlorinated side reactions and thus improve cyclohexanone selectivity. Based on these findings, a free-radical mechanism initiated by the Cl atoms generated in the excited POM-((VOHM)-O-V)Cl-+(-) species was proposed. (C) 2017 Elsevier B.V. All rights reserved.