The molecular conformation of the high-energy isomer of methylvinylether has been re-investigated. Ab initio calculations were performed using the 6–31G and 6–31G* bases at the SCF level, accounting for electron correlation by full second- and third-order Møller—Plesset perturbative treatments. The internal potential energy function, as derived from calculations at several torsional angles with complete geometry optimization, indicated the high-energy isomer in a wide double-minimum potential well at a CCOC dihedral angle between 156.7 and 159.4° from the most stable s-cis form. At every level the two equivalent minima were scarcely resolved, as expressed by a very low interconversion barrier ranging from 10.8 to 21.7 cm −. Estimates of the skeletal torsional energies in the one-vibration approximation gave aground state energy at or just above the barrier height, thus depicting the high-energy isomer as an effective s-trans form. The IR spectra of the pure liquid and the vapour were taken at several temperatures. In the mid-IR spectrum at medium-high resolution, marked fine rotational structure appeared in almost all of the gas-phase absorption envelopes of both isomers.Four bands attributed to the high-energy isomer were analyzed inthe approximation of a quasi-prolate rotor: one, of a parallel-like type, yielded B+ C=0.28 −1; the other three, of perpendicular-like type, yielded similar values, 2.38, 2.34 and 2.27 cm −1, for 2[A—(B+ C)/2]. Both these molecular parameters agree with those expected for an s-trans conformation or one very near to it. A prediction of the vibrational spectrum of the high-energy isomer was made from the harmonic force constants derived at the SCF 6–31G* level; the calculated frequencies were corrected by transferring scale factors from the s-cis isomer. With this procedure, the average difference between the eleven observed frequencies and the corresponding predicted frequencies amounted to 4.2 cm −1.