A formal platinum(V) dioxide cation [Pt,O2](+) can be generated in the gas phase by successive oxidation of Pt(+) with N2O. The ion's reactivity is in keeping with the dioxide structure OPtO(+), rather than with [Pt,O2](+) isomers having intact O-O bonds, e.g., the dioxygen complex Pt(O2)(+) and peroxo species PtOO(+). Inter alia due to the high ionization energy of the neutral counterpart (11.2 eV), the [Pt,O2](+) cation is a rather aggressive reagent toward oxidizable neutrals. [Pt,O2](+) is even capable of activating inert substrates such as H2, CO, and CH4. Further, a sequence for the catalytic conversion CO + N(2)O --> CO2 + N2 is described with a turnover number of >100 for the catalytically active species PtOn(+) (n = 0-2). As a consequence of the high reactivity, however, the observed selectivities with most substrates are rather poor. For example, the reaction of PtO2(+) with ethane gives rise to 10 different product channels. In an attempt to analyze the structural features and different minima of the [Pt,O2](+) system, extensive ab initio studies are performed. While correlated ab initio methods describe the system reasonably well, density functional theory turns out to be much less accurate in terms of both structural and energetic descriptions.