Ab initio molecular orbital calculations have been carried out to examine the relative stabilities and the spectroscopy of isomers of HClO3. The lowest energy structure is HOClO2, and its heat of formation at 0 K is calculated to be 4.2 kcal mol(-1) as determined from G2 theory. The next lowest energy isomer is a skewed-straight chain HOOOCl. It is 4.9 kcal mol(-1) above the lowest energy form. The two higher energy structures are HOOClO and the hypervalent HClO3. An examination of the vibrational and electronic spectra reveals spectral features that allow these species to be uniquely distinguished.