Molecular silicon trioxide has been isolated in argon matrices at low temperature (i) as a secondary reaction product within Si+2(O-2) aggregates or (ii) as a primary product in the direct reaction of silicon monoxide with oxygen molecules after UV excitation at 5.17 eV. The silicon trioxide molecule has been characterized through the observation of five fundamental vibrations for at least eight different isotopic species, namely (SiO3)-Si-28-O-16, (SiO3)-Si-29-O-16, (SiO3)-Si-30-O-16, (OSiO2)-O-16-O-18, (OSiOO)-O-18-O-16-O-18,(OSiOO)-O-16-O-16-O-18, (OSiO2)-O-18-O-16, and (SiO3)-O-18. From the experimental data, a structure of the C-2v symmetry can be deduced with a Si=O double bond much shorter than two equivalent Si-O single bonds. These findings are fully supported by DFT calculations, which moreover predict a large binding energy of about 60 kcal/mol with respect to the separate fragments (SiO and O-2) The possible reaction pathways are also discussed.