The diurnal variation of BrO through sunrise and sunset has been measured above Cambridge, England, (52¿N) during March and April 1994 using zenith sky spectroscopy. The measured BrO slant column at 90¿ solar zenith angle (SZA), relative to that at 80¿ SZA, is typically 1.5¿1014 cm-2 but varies significantly from day to day. The average variation of BrO slant column with SZA through sunrise and sunset is in good agreement with predictions from a photochemical model, coupled to a radiative transfer model. The zenith sky measurements are consistent with in situ measurements of BrO concentrations and indicate that the inorganic bromine content of the stratosphere is 18--24 parts per trillion by volume (pptv). ¿ American Geophysical Union 1995