Twilight sky colour measurements were made using ground-based UV-visible SAOZ spectrometers at four stations along the Arctic circle during EASOE. The results show that volcanic aerosol from the Pinatubo eruption of June 1991 first appeared above the Arctic in September 1991. The aerosol layer thickened progressively during the autumn and had spread to all four stations, outside in the very low stratosphere, as well as inside the polar vortex, by mid-January. For the rest of the campaign, to mid-March, little further change was seen. The aerosol would have masked any PSCs that may have occurred at altitudes less than 22 km. However, PSCs at higher altitudes should have been detected, had their optical thickness exceeded 0.01. Only one was recorded during EASOE: on 28–29 December 1991, in contrast to their frequent occurrence in January and early February 1990.