Zenith-sky observations of the seasonal evolution of BrO slant columns above Aberdeen (57 ¿N) between November 1994 and April 1995 are presented. The measurements show maximum differential BrO slant columns (90¿--81¿) of 2.4¿1014 molecules cm-2 in January, decreasing to 1¿1014 in spring. On a seasonal timescale, BrO slant columns are anticorrelated with NO2 slant columns, which is qualitatively consistent with BrONO2 being the main night-time bromine reservoir. There is some evidence for a small (5¿1013 molecules cm-2) increase in sunrise BrO slant columns inside the polar vortex, following chlorine activation on PSCs, as shown by measurements of large OClO slant columns.¿ 1997 American Geophysical Union