Photoionization efficiency (PIE) spectra of HOI were measured over the wavelength range lambda = 115-130 nm and in the ionization threshold region, lambda = 123-129 nm, using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. HOI was generated, in situ but in varying amounts, by three separate reactions : OH + I-2; OH + CF3I; O(P-3) + C2H5I. The PIE spectra displayed steplike behavior near threshold, and the HO-I stretching frequency in the cation was determined to be 702 +/- 60 cm(-1). A value of (9.81(1) +/- 0.02(0)) eV was obtained for the adiabatic ionization energy (IE) of HOI from photoion thresholds, corresponding to the HOI+(X(2)A ") <-- HOI(X(l)A’) transition. Even though the present result appears to be the first reported determination of IE(HOI), the experimental value is compared to an estimated value previously derived via a trend analysis and it is considered in terms of trends in the series IE(HOX), where X = F, Cl, Br, and I. The branching ratio for [HOI]/[IO] in the O(P-3) + C2H5I reaction was estimated to be about 8/1 at T = 298 K if we assume that the photoionization efficiencies for HOI and IO are the same at 10.3 eV (lambda = 120 nm). Also, based on the value for IE(HOI) derived in the present study, a value for IE(IO) approximate to 9.66 +/- 0.10 eV has been predicted.