The photoionization efficiency (PIE) spectrum of Br2O was measured over the wavelength range lambda = 100-125 nm by using a photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. A value of 10.26(4) +/- 0.01(1) eV was obtained for the adiabatic ionization energy of Br2O from analysis of the photoionization thresholds. Also, the appearance energy (AE) of BrO+ (from the dissociative ionization of Br2O) was determined from the PIE spectrum of BrO+ over the wavelength range lambda = 104-110 nm. Combining the AE(298) value, 11.71(4) +/- 0.01(4) eV, with known thermodynamic quantities yields a value for Delta(f)H degrees(298)(Br2O) = 107.1 +/- 3.5 kJ mol(-1). The value for Delta(f)H degrees(298)(Br2O) may be related to Delta(f)H degrees(298)(HOBr) through the equilibrium constant for the reaction Br2O + H2O reversible arrow 2HOBr. This leads to Delta(f)H degrees(298)(HOBr) = -60.0 +/- 3.5 kJ mol(-1).