A high-resolution (1-7 meV) threshold photoelectron spectroscopic study of IBr was performed using synchrotron radiation and a penetrating-field electron spectrometer over the valence ionization region of the molecule. Extensive vibrational structure was found in all three electronic-state band systems (X(2)Pi(i), A(2)Pi(i) and B(2)Sigma(+)) of IBr+. In the (X(2)Pi(i)) band system both spin-orbit components exhibited extended vibrational structure in the Franck-Condon gap regions that is attributed to resonance autoionization of neutral Rydberg states lying in these energy regions. Analysis of this vibrational structure yielded accurate spectroscopic constants. (C) 2003 Elsevier Science B.V. All rights reserved.