The threshold photoelectron spectrum (TPES) of tetrahydrofuran (THF) is compared to that of the unsaturated furan molecule. In general, there is a similarity in the orbital ionization profile for the two species, though unlike furan, THF exhibits (modest) vibrational detail only in the (9b)(-1) (X) over tilde B-2 hand. An adiabatic ionization energy of 9.445 +/- 0.010 eV has been derived from the onset of the TPES spectrum. Threshold photoelectron photoion coincidence spectroscopy Was used to explore the loss of a hydrogen atom from ionized THF over the photon energy range of 9.9-10.4 eV. RRKM fitting of the resulting breakdown Curves yields an E-0 of 0.85 +/- 0.03 eV (82 +/- 3 kJ mol(-1)) (AE = 10.30 +/- 0.04 eV). If the G3 IE of 9.48 eV is used to convert the experimental data from photon energy to THF ion internal energy, E-0 = 0.81 +/- 0.01 eV (78 +/- 1 kJ mol(-1)). The latter value is closer to the G3 E-0 of 72 kJ mol(-1) for the formation of the cyclic ion 1. A variety of ring-opening reactions were also probed at the B3-LYP/6-31+G(d) and G3 levels of theory. The distonic isomer (CH2CH2CH2OCH2+)-C-center dot lies 70 kJ mol(-1) higher than ionized THF, which places it within 1 kJ mol(-1) of the threshold for the dissociation to 1. All of the probed H-loss products from the distonic isomer (which includes singlet and triplet species) lie significantly higher in energy than ion 1, eliminating the possibility that ionized THF dissociates to m/z 71 via a ring-opening reaction in the present experiment. The derived Delta double dagger S value for the dissociation, 8 +/- 5 J K-1 mol(-1), is also consistent with the formation of 1. The experimentally derived E-0 values can be used to derive the Delta H-f(0)degrees for ion 1. Together with the Delta H-f(0)degrees values for the THF ion (752.0 +/- 2 kJ mol(-1), derived from the neutral Delta H-f(0)degrees of -154.9 +/- 0.7 kJ mol(-1) and experimental IE of 9.445 +/- 0.010 eV) and H atom (218.5 kJ mol(-1)) our E-0 of 82 +/- 3 kJ mol(-1) yields a Delta H-f(0)degrees for ion 1 of 620 +/- 4 kJ mol(-1) (Delta H-f(298)degrees = 594 +/- 4 kJ mol(-1)), in good agreement with the G3 Delta H-f(0)degrees of 621 kJ mol(-1). Appearance energies for all fragment ions up to photon energies of 34 eV are also reported and discussed in comparison with the available literature.