The photoionization efficiency (PIE) spectrum of Cl2O was measured over the wavelength range lambda = 98-117 nm by using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. A value of 10.90(9) +/- 0.01(6) eV was obtained for the adiabatic ionization energy (IE) of Cl2O from analysis of the photoionization thresholds. The ClO radical was produced via the reaction F + Cl2O --> FCl + ClO and the PIE spectrum of ClO was measured over the wavelength range lambda = 105-117 nm. Analysis of the photoionization threshold yields a value of IE of ClO(X(2) Pi(3/2)) = 10.88(5) +/- 0.01(6) eV. The appearance energy (AE) of ClO+ (from the dissociative ionization of Cl2O) was determined from the PIE spectrum of ClO+ over the wavelength range lambda = 98-104 nm. Combining the AE(298) value, 12.29(6) +/- 0.03(2) eV, with known thermodynamic quantities yields a value for Delta(f)H(298)(degrees)(Cl2O) = 77.2 +/- 3.4 kJ mol(-1). From the value for Delta(f)H(298)(degrees)(Cl2O) and the equilibrium constant for the reaction Cl2O + H2O <----> 2HOCl, a value for Delta(f)H(298)(degrees)(HOCl) = -76.8 +/- 3.5 kJ mol(-1) is obtained. These results for IE(Cl2O), IE(ClO), AE(298)(ClO+, Cl2O), Delta(f)H(298)(degrees)(Cl2O), and Delta(f)H(298)(degrees)(HOCl) are compared with previous estimates or measurements.