Gas-phase experiments are performed aimed at a structural characterization of SiO3.+, which is-as reported earlier [(a) Creasy, W. R.; O’Keefe, A.; McDonald, J. R. J. Phys. Chem. 1987, 91, 2848. (b) Wlodek, S.; Bohme, D. K. J. Chem. Sec., Faraday Trans. 2 1989, 85, 1643.]-readily available by oxidizing SiO2.+ with N2O, and the combined experimental/theoretical results point to the formation of the covalently bonded trioxide 1. The absence of oxygen exchange in the thermoneutral reaction of SiO3.+ With O-18(2) as well as collision-induced dissociation (CID) experiments of SiO3.+ rule out definitively a loosely bound ion-dipole complex [O2SiO2](.+) for the initially generated SiO3.+ radical cation. In the near-thermal ion-molecule reactions of SiO3.+ with X (X = H2O, N2O, CH3CN, and C2H4), the SiO(X)(.+) products are formed with collision rate, and the kinetic isotope effects are negligibly small (1.1-1.3) when D2O, CD3CN, and C2D4 are employed. The product SiO((H2O)-O-18)(.+), generated from SiO3.+ and (H2O)-O-18, upon collisional activation gives rise to SiOH+ and (SiOH+)-O-18 in a nearly 1:1 ratio, thus pointing to a symmetrical intermediate. From bracketing experiments the standard enthalpy of formation of SiO3.+ is estimated as 186 < Delta H degrees(f,298)(SiO3.+) < 212 kcal/mol. Preliminary ab initio MO calculations (MP4SDTQ/6-31G*//MP2(Full)6-31G*) of several SiO3.+ isomers provide the following information : The (4)A(1)’ State of the D-3h-symmetrical structure 1 is found to correspond to the global minimum, and the slightly distorted C-2 upsilon structure 2 ((2)A(1)) is 11.2 kcal/mol less stable than 1. The weakly bound ion-dipole complexes 5 and 6, which may serve as intermediates in the collision-induced dissociation of 1, are only destabilized by 2.2 and 5.5 kcal/mol relative to 1. For the C-2 upsilon-symmetric structures 3 and 4, there is no computational evidence that these radical cations exist as minima on either the doublet or quartet potential energy surfaces of SiO3.+.