A new SixOy species has been isolated in argon matrices at low temperatures by Roy et al. [J. Chem. Phys. 104 (1996) 8]. The new species was characterized by vibrational spectra and proposed to be SiO3 in a C-2v structure, with an unusual O-O bond. To explore this structure, we use the CCSD(T), QRHF-CC, EOM-CC, IP-STEOM and the DIP-STEOM method to investigate SiO3 and its anion. The IP-STEOM and DIP-STEOM schemes allows us to describe traditional multi-reference situations in what is conceptionally a one determinantal frame work. Our results support the experimentally found C-2v structure. (C) 2002 Elsevier Science B.V. All rights reserved.