AbstractNew measurements of the depolarization ratios of theRamanlines of liquid trimethylene oxide and more accurate values of the rotational constants suggested a new assignment of the vibrational spectra of this molecule. A normal coordinate treatment has been carried out, showing good agreement with experimental data for the typical CH2‐group vibrations. For the strongly coupled ring vibrations much less satisfactory results were obtained from conventional values of force constants. The newly exposed assignment differs in several respects from previous work. For the low frequency non planar ring vibration ν24two values of frequency were discussed, but no definitive choice has been achieved. The thermodynamic functions for the ideal gas have been recalculat