Eleven new polyoxoanion-supported transition metal acetonitrile compositions have been synthesized in 1:1, 1:2, and even a few 1:3 Mn+ to polyoxoanion ratios; these new precatalysts were then tested for their catalytic efficacy for norbornene and cyclohexene oxygenation using [PhIO](n) as the oxidant. The catalytic results identify {(CH3-CN)(x)Mn2+/P2W15Nb3O629-} (1) and {2(CH3CN)(x)Mn2+/P2W15Nb3O629-} (2) as the best precatalysts within this new subclass of polyoxoanion-supported catalysts. The results reveal a modest ca. 14-fold rate increase for either 1 or 2 in norbornene or cyclohexene epoxidation compared to the polyoxoanion-free [Mn-II(CH3CN)(4)](BF4)(2) solvate, kinetic results which require the presence of the P2W15Nb3O629- polyoxoanion in the rate-determining step of the active catalyst. Catalyst reisolation, then LR, UV-visible, and ion-exchange resin studies (and in comparison to authentic 2 that has not undergone catalysis) provide compelling evidence that 2 is, in fact, the true catalyst. Also compared to 1 and 2 is the polyoxoanion-framework-incorporated (P2W17MnO617-)-O-III, the first such comparison of a polyoxoanion-supported and -incorporated catalyst; the results reveal, for example, a distinctive catalytic epoxidation stereochemistry for the polyoxoanion-supported vs the polyoxoanion-incorporated Mn catalysts. These experiments, only the second test of the concept of polyoxoanion-supported catalysis, provide further evidence for the conceptual distinctiveness of polyoxoanion-supported transition metals as a new subclass of all-inorganic, oxidation-resistant, polyoxoanion-based catalysts.