Equilibrium geometry, force constants, vibrational frequencies, and infrared intensities of the M₂XO­₃ (M = Li, Na, and K and X = S, Se, and Te) molecules are calculated by the ab initio Hartree-Fock method using double-zeta valence triple-zeta basis sets augmented by the polarization and diffuse functions and with relativistic effective core potentials. The relative energies of different molecular configurations and energies of the M₂XO₃ ==> M₂O + XO₂ dissociation are calculated with second-order perturbation theory. The chemical bonding in these molecules can be approximately formulated as (M⁺)₂[XO₃]^2-. The equilibrium nuclear configuration (C_s symmetry) corresponds to the bisbidentate coordination of the M⁺ cations by the pyramidal XO₃^2- anion (bb). The monodentate-bidentate structures (mb) of symmetry C_s correspond to the saddle points in the potential energy surfaces of the molecules. The results of infrared spectrum calculations are in satisfactory agreement with the experimental spectra of the matrix-isolated Na₂SeO₃, K₂SeO₃ and K₂TeO₃ molecules. The trends in changing the molecular parameters are revealed for the Li₂XO₃ ==> Na₂XO₃ ==> K₂XO₃ sequence and for the M₂SO₃ ==> M₂SeO₃ ==> M₂TeO₃ series. The barrier of the intramolecular rearrangement (bb)==>(mb)==>(bb)' was found to lower in the series Li₂XO₃==>Na₂XO₃==>K₂XO₃ (X = S, Se, Te) and M₂TeO₃==> M₂SeO₃==>M₂SO₃. The degree of deformation of the XO₃^2- moiety in the M₂XO₃ molecules decreases if the M atom is replaced by its heavier analog (Li==>Na==>K).

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