The geometrical structure, force fields, and vibrational spectra of the ClO 3 − ion and LiClO3, NaClO3, and KClO3 molecules are studied using the Hartree-Fock (HF) method and second-order Moller-Plesset (MP2) perturbation theory in double-zeta basis sets complemented with polarization and diffuse functions. Routes of intramolecular rearrangements corresponding to migration of the M+ cations around the ClO 3 − anion are investigated. The calculations showed that the molecular structure of alkaline metal chlorates changes in the series LiClO3 → NaClO3 → KClO3. The LiClO3 molecule has an essentially bidentate configuration of Cs symmetry; the KClO3 molecule has tridentate coordination of C3v symmetry. The NaClO3 molecule exists as two isomeric forms having similar energies: tridentate (C3v) and bidentate (Cs) forms separated by a low potential barrier (199 cm−1 − HF, 170 cm−1 − MP2); the energy differences between the isomers are ΔE(Cs − C3v)=−0.5 (HF), 0.4 kJ/mole (MP2). The theoretical vibrational spectra of molecules agree with the available experimental data.