Photoionization efficiency (PIE) spectra of IO(X(2) Pi(i)) were measured over the wavelength range lambda = 115.0-130.0 nm and in the ionization threshold region lambda = 126.0-130.0 nm, using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. Iodine oxide was generated by the reactions of O(P-3) atoms with I-2 and CF3I. The PIE spectra displayed step-function behavior. From the half-rise point of the initial step, a value of 9.73(5) +/- 0.01(7) eV was obtained for the adiabatic ionization energy IE) of IO, corresponding to the IO+(X(3) Sigma(-)) <-- IO(X(2) Pi(3/2)) transition. As this appears to be the first experimental determination of IE(IO), a trend analysis has been employed to provide further insight into the experimentally derived value. The separation between the first two steps in the PIE spectrum (which might be perturbed slightly due to autoionization) was used to derive the vibrational spacing of the cation, 1060 +/- 160 cm(-1). A Rydberg progression, presumably converging to the excited (1) Delta state of the cation, has a convergence limit that lies 0.596 eV above the ground state of the cation. A selected value for the bond dissociation energy of IO, D-0(o)(IO) = 226 kJ mol(-1), leads to Delta(f)H(0)(o)(IO) = 128 i 4 kJ mol(-1). From this value for the heat of formation of IO, estimates are made for Delta(f)H(0)(o)(IO+), Delta(f)H(0)(o)(HOI) and the proton affinity of IO.