He(I) photoelectron (pe) spectroscopy is applied to determine the preferred gas phase conformations of a limited number of flexible allylic ethers and alcohols. Based on earlier observations that the π-ionization energy is increased more when the allylic C—O bond is coplanar (with the π-system) than when it is perpendicular, the pe spectrum of cis and trans-4-tert-butyl-2-cyclohexanol and their corresponding ethers, and 5α-hydroxy(and methoxy)-10α-Δ3-octalin have been determined. The results indicate that when a coplanar arrangement of the allylic C—O bond can be attained, it is preferred, leading to a favored conformation of the allylic alcohol or ether.