The 0–1 transition of the out-of-plane bending vibration in trimethylene oxide has been observed at 53.4 cm−1. Previously reported bands by Lord and his co-workers [J. Chem. Phys. 33, 294 (1960)] at 89.8, 105.2, 118.3, 128.9, 139.0, 147.6, 154.9, and 161.8 cm−1 have also been confirmed. In addition, six higher members of this series as well as three members of the Δv = 3 series have been observed. These transitions are readily interpreted in terms of the hot-band series of a slightly perturbed quartic oscillator. A double-minimum potential function of the form az4—bz2 predicted all the observed transitions to within experimental error (0.5 cm−1 or better). The central barrier in the potential-energy function, while low, is finite and has a height of 15.3±0.5 cm−1. It had previously been concluded, from the variation of the rotational constants with vibrational state obtained from microwave spectroscopy that the potential energy contained a small barrier of the order of zero-point energy at the planar configuration of the molecule [J. Chem. Phys. 33, 1643 (1960)]. The present far-infrared results are shown to be consistent with the microwave results. Thus an existing discrepancy between the far-infrared and microwave results on the nature of the potential function for this unique out-of-plane bending vibration is finally resolved.