Seven isotopomers of the hydrogen-bonded heterodimer tetrahydrofuran ... HF have been investigated in the frequency range 6-18 GHz using molecular-beam Fourier transform microwave spectroscopy. The rotational spectra of C4H8O ... HF and C4H8O ... DF exhibit small tunneling splittings, which were not observed for C4D8O ... HF and the four single C-13 isotopomers. These observations analyzed in terms of symmetry considerations allow us to conclude that these splittings are due to pseudorotation within the tetrahydrofuran subunit of the complex and not to HF inversion. The spectroscopic parameters of the complex have been interpreted in terms of a geometry in which tetrahydrofuran has a conformation close to the twisted ring form, with HF lying on the plane bisector to the COC ring angle.