The gas-phase UV/visible absorption spectrum of HOBr, an important atmospheric trace species, is reported for the first time. The HOBr spectrum, measured over the range 200-420 nm, consists of two absorption bands peaking near 280 nm (sigma(max) = (3.1 +/- 0.4) x 10(-19) cm(2) molecule(-1)) and near 350 nm (sigma(max) = (6.1 +/- 1.0) x 10(-20) cm(2) molecule(-1)). Atmospheric photolysis rates for HOBr calculated from these data are found to be significantly slower than previously assumed. The UV/visible absorption spectrum of Br2O, which was used as the HOBr precursor, was also recorded for the first time. The Br2O spectrum has maxima near 200 nm (sigma(max) = (2.0 +/- 0.2) x 10(-17) cm(2) molecule(-1)) and 314 nm (sigma(max) = (2.3 +/- 0.3) x 10(-18) cm(2) molecule(-1)). These spectra of Br2O and HOBr are compared with those of the analogous chlorine species. In addition, some thermodynamic properties for Br2O and HOBr have been obtained. The equilibrium constant for the reaction Br2O + H2O <-> 2 HOBr (10, -10), was determined to be 0.02, yielding Delta G(r)(o)(298 K) = 2.33 kcal/ mol for the forward reaction. This equilibrium constant is evaluated in terms of recent reports of Delta H-f(298 K) for HOBr.