Threshold photoelectron spectra of the tetrahydrofuran (THF) molecules, C 4 H 8 O have been measured over the photon energy range 9–29eV using synchrotron radiation. For the lowest photoelectron band at 9.3–10.5eV, due to ionization from the oxygen atom lone-pair 9b/12a′ orbital (C 2 /C s conformations), vibrational structures have been observed superimposed on a broad maximum. The resolved vibrational series have been attributed to ionization transitions from the C 2 conformation of the THF molecule with an adiabatic energy of 9.427eV. The underlying broad maximum has been assigned to ionization from the C s conformation of the THF molecule with a vertical energy of 9.73eV. This is the first observation of the ionization transitions from two conformations of a pseudorotating molecule. New photoelectron bands have been also identified in the photon energy region 16–26eV.