Iodine oxides appear as reactive intermediates in atmospheric chemistry. Here, we investigate IO and HOI by mass-selective threshold photoelectron spectroscopy (ms-TPES), using synchrotron radiation. IO and HOI are generated by photolyzing iodine in the presence of ozone. For both molecules, accurate ionization energies are determined, 9.71 � 0.02 eV for IO and 9.79 � 0.02 eV for HOI. The strong spin-spin interaction in the 3 Σ À ground state of IO + leads to an energy splitting into the Ω = 0 and Ω = � 1 sublevels. Upon ionization, the IÀ O bond shortens significantly in both molecules; thus, a vibrational progression, assigned to the IÀ O stretch, is apparent in both spectra.