A balloon-borne UV--visible spectrometer, the SAOZ-BrO, has been designed for the measurement of BrO on small and relatively low-cost balloons. It allows the retrieval of the vertical BrO profile with a resolution of 1 km, a precision of 0.5--2 pptv (below 25 km), and a +5/-10% accuracy during the daytime balloon ascent. Fifteen successful flights have been carried out since 1997. Significant BrO amounts were observed at all latitudes and seasons, with a peak concentration altitude varying from 15 km in the winter vortex to 22 km in the tropics. The mixing ratio increases steadily from the tropopause to 25--30 km, depending on the latitude, above which it remains constant up to 30 km. The latitudinal and seasonal changes (maximum at high latitude and in the winter) are largely controlled by the vertical transport of total inorganic bromine and to a smaller extent by photochemistry. Photochemical changes are primarily related to NO2 abundances. On a constant potential temperature surface, the BrO mixing ratio is the largest in Polar Regions in the winter, where NO2 is nearly absent. In contrast, BrO is the smallest during the polar day and in the summer at midlatitude. The presence of activated chlorine in the cold vortex has little impact on BrO abundances. Finally, significant amounts were observed in the upper troposphere: (1) in the summer at midlatitude where it was the result of a stratosphere--troposphere exchange (STE) event advecting bromine from the stratosphere and (2) at the tropics where its presence is likely due to the conversion of organic bromine at lower altitude.