The absorption cross-section of gaseous HOBr was determined over the wavelength range 235 to 430 nm with a spectral resolution of 0.6 nm full width at half maximum (FWHM) using a diode array spectrometer. The spectrum of HOBr shows two main absorption bands with maxima near 282 nm <&sgr;=(3.1¿0.4)¿10-19 cm2 molecule-1 and 350 nm (&sgr;=12.5¿1.6)¿10-20 cm2 molecule-1> extending out to 430 nm. The absorption cross-sections in the first absorption band are in good agreement with a recent determination; the cross-sections in the second band however, are approximately a factor of 2.5 larger than previously determined. In addition we provide evidence in support of a weak band in HOBr around 440 nm (&sgr;≈7.5¿10-21 cm2 molecule-1) as observed by Barnes et al. <1996>. The absorption cross-section of Br2O, which was used to prepare HOBr, was determined over the wavelength range 230 to 750 nm. The spectrum shows four absorption bands with maxima at 314 nm <&sgr;=(2.1¿0.3)¿10-18 cm2 molecule-1>, 350 nm <&sgr;=(1.9¿0.2)¿10-18 cm2 molecule-1>, 520 nm <&sgr;=(4.4¿0.5)¿10-20 cm2 molecule-1>, and 665 nm <&sgr;=(6.2¿0.9)¿10-20 cm2 molecule-1>. The visible bands at 520 nm and 660 nm have not been observed previously. The equilibrium constant, for the reaction Br2O+H2O⇔2HOBr was determined to be 0.037¿0.004 at 298 K. Measurement of the equilibrium constant as a function of temperature enabled values for ΔH298 K=(13.0¿0.5) kJ mol-1 and ΔS298 K=(16¿2) J mol-1 K-1 to be determined. The absorption cross-section data for HOBr have been used in a photochemical box model to investigate the significance of these results in the lower stratosphere. The model results are compared with observations during a recent Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE) and show that the revised HOBr cross-section, coupled to the rapid heterogeneous conversion of BrONO2 to HOBr, can account quantitatively for the abrupt morning rise in HOx. ¿ American Geophysical Union 1996