The far-infrared spectra of bicyclo(3.1.0.)hexane, 3-oxabicyclo(3.1.0.)hexane, and 3,6-dioxabicyclo(3.1.0.)hexane exhibit series of Q branches in the frequency range 250–125 cm−1, like those observed for 6-oxabicyclo(3.1.0.)hexane (cyclopentene oxide) by Carreira and Lord (J. Chem. Phys. 51, 2735 (1969)). The Q branches are interpreted as single quantum jumps of a one-dimensional ring-puckering vibration governed by a potential function of the form V(cm−1) = A(Z4 + BZ2 + CZ3), where Z is a reduced ring-puckering coordinate.The potential parameters for bicyclo(3.1.0.)hexane are A, 24.31 cm−1; B, 26.67; C, 9.63; for 3-oxabicyclo(3.1.0.)hexane, 27.77 cm−1, 20.63, 7.84; and for 3,6-dioxabicyclo(3.1.0.)hexane, 25.25 cm−1, 17.23, 7.33. They were determined by an iterative least-squares fit to the observed four, seven, and nine Q branches, respectively. These potential functions all have only a single minimum, implying a single stable conformation. The dipole moment, μrms, for 3,6-dioxabicyclo(3.1.0.)hexane in benzene solution was determined to be 2.50 D, clearly indicating that the stable conformation is the boat form. The boat conformation had been determined to be the stable form for cyclopentene oxide from a microwave study by Lafferty (J. Mol. Spectrosc. 36, 84 (1970)). There is no direct evidence for the preferred conformation of the remaining two molecules but consideration of torsional interactions about the 1–2 and 4–5 bonds as well as the similarity of the spectra and potential functions for these molecules suggests that the boat conformation is the stable form of all of these molecules.