A combination of steady-state and time-resolved techniques has been used to investigate the mechanism of the decatungstate (W10O324-) photocatalyzed oxygenation of cyclohexene, and of a series of hexene isomers, as well as of their saturated counterparts, in acetonitrile. It is demonstrated that (i) alkenes react by both charge-transfer and hydrogen-atom-transfer mechanisms, the former being dominant in each case; (ii) alkanes exclusively react by hydrogen-atom abstraction; (iii) a previously unknown reaction pathway, resulting from interception of the geminate radical ion pair by molecular oxygen, and leading to quantitative formation of hydroperoxides without decatungstate reduction, can significantly contribute to the overall reaction; (iv) no back-electron transfer occurs prior to separation of the geminate pair or its interception by 02; (v) the catalytic activity is strongly reduced in the presence of olefins, due to the formation of very long-lived complexes involving both one- and two-electron-reduced decatungstate; and (vi) the decatungstate anion undergoes strong precomplexation with the reactive solvent acetonitrile.