A high-resolution (1-11 meV) threshold photoelectron spectroscopic study of ICl has been performed using synchrotron radiation and a penetrating-field electron spectrometer over the valence ionization region of the molecule. Accurate vibrational constants have been obtained for both spin-orbit components of the ICl+ (X(2)Pi(i)) state through the observation of extended vibrational bands in the Franck-Condon gap regions of the molecular ion. The appearance of these vibrational bands is attributed to resonant autoionization of Rydberg states residing in this energy region of the ionic state. The adiabatic (nu(+) = 0) ionization potentials for formation of the ground state of the ICl+ ion are found to be 10.076 +/- 0.002 eV for (X(2)Pi(3/2)) and 10.655 +/- 0.002 eV for (X(2)Pi(1/2)) yielding a spin-orbit splitting in this state of 0.579 +/- 0.002 eV. The adiabatic (nu(+) = 0) ionization potential for formation of the (A (2)Pi(i)) state of ICl+ is estimated to be at similar to 12.5 eV. Additional autoionization effects are noted in the formation of the (A(2)Pi(i)) and (B(2)Sigma(+)) band systems in ICl+.