BrO measured from the NASA ER-2 during AASE II exhibited a mean value (for 20-minute averages) of 5.4¿0.3 pptv, with a standard deviation of 3.1 pptv. Ratios of BrO to available inorganic bromine (Bry) show only slight increases in polar regions relative to midlatitudes. A comparison between observed latitudinal and diurnal variations of this same ratio and that calculated by photochemical models shows reasonable agreement in behavior, but significant discrepancies in magnitude. It is unclear whether this difference is due to errors in measurements, models or both. ¿American Geophysical Union 1995