Microwave spectra have been measured for the planar molecules 4H-pyran-4-one (I), 4H-pyran-4thione (11), 4H-thiapyran-4-one (111) and 4H-thiapyran-4-thione (IV). Extensive isotopic data for I, I1 and IV enable precise, complete structures to be calculated. There is little structural evidence for enhanced aromaticity in these molecules.The high, almost constant, values for the dipole moments near 4 D, previously indicated from solution measurements on I, I1 and IV, have been confirmed and the values more closely defined.Vibrational satellite spectra indicate the lowest frequency fundamentals in these molecules to be out-of-plane motions near 100 cm-'.