The appearance energy of BrO+ from the dissociative ionization of OBrO was determined from the photoionization efficiency spectrum of BrO+ over the wavelength range lambda = 90-122 nm by using a discharge flow-photoionization mass spectrometer apparatus coupled to a vacuum- ultraviolet synchrotron radiation source. Bromine dioxide was generated in a flow tube reactor by first forming BrO via the reaction O(P-3) + Br-2 and then allowing the BrO to react on the cold flow tube wall. Species present in addition to OBrO were BrO, HOBr, Br2O, and residual BT2. The results are perturbed by a signal from BrO+ formed by direct photoionization of BrO and by the dissociative ionization of Br2O. It was possible to correct for these perturbations to obtain a clean appearance energy plot that yielded a threshold at lambda = 98.65 +/- 0.23 nm. This gives AE(298)(BrO+,OBrO) = 12.56(8) +/- 0.02(9) eV. Taking known thermodynamic quantities, this result yields Delta H-f(0)o(OBrO) = 173.4 +/- 4.3 kJ mol(-1) (and Delta H-f(298)o(OBrO) = 163.9 +/- 4.4 kJ mol(-1)). This is the first experimental determination of the heat of formation of OBrO, and the present result is compared with those of previous estimates and recent calculations. In addition, computations (based on results from ab initio calculations) were performed to obtain Delta H-f(0)o(OBrO) = 164 +/- 8 kJ mol(-1), which is in very good agreement with our experimental result. Also, a value for Delta H-f(0)o(OIO) of 174 +/- 25 kJ mol(-1) was estimated from a trend analysis that employed Chase's method (i.e., Delta H-at(0)o(OXO)/D-0(o)(XO)). Additionally, a comparison is made of recent photoelectron spectroscopic and photoionization mass spectrometric determinations of the ionization energies of BrO and OBrO.