Solid samples of alkali metal selenates (M2SeO4) and selenites (M2SeO3) have been heated in vacuo, and the vaporization products studied by using a combination of mass spectrometric and matrix isolation IR techniques. Although some decomposition was observed, evidence was obtained for the existence of the ternary molecular salts M2SeO4 and M2SeO3 (M = Na, K, Rb, Cs) and also for the novel Se(III) species MSeO2. The molecular symmetries are identified as D2d, C(s), and C2v, respectively, on the basis of vibrational selection rules, and the various O-Se-O bond angles in these species are estimated from selenium isotope shifts and relative band intensities.