Eight years of continuous measurements of the ozone column at Thule, Greenland are presented. Monthly average values of the ozone column have been observed to decrease significantly in spring during this decade. Lower ozone columns with large interannual variations have been observed when the polar vortex was above the site. In 1995 and 1996 March monthly mean values were very low with an average deficit in the vertical column of more than 30% or 150 DU, when considering only days where the site was inside the vortex. There are two contributions to the changes in the total column ozone one is transport and the other is chemical depletion. From the variation in the geopotential height of 250 hPa level and the temperature at the 475 K isentropic level an estimate of the part of the decrease in the vortex March monthly mean ozone column which could be explained by vertical transport was obtained. It was found that vertical transport could explain a 10% decrease in 1995 and a 5% decrease in 1996, leaving 25% to be explained by chemical ozone destruction. ¿ 1999 American Geophysical Union