The time evolution and latitudinal distribution of ozone and chlorine species (HCl, ClONO2, and ClO) in the Arctic and midlatitude stratosphere have been studied for the winter of 1999/2000. This has been done by the use of ground-based solar absorption Fourier Transform Infrared (FTIR) measurements from four locations: Harestua (60.2¿N, 10.8¿E), Kiruna (67.8¿N, 20.4¿E), Esrange (67.9¿N, 21.1¿E), and Ny ¿lesund (79¿N, 12¿E). The measurements were normalized with the inert tracer HF to minimize the effects of transport, combined using the concept of equivalent latitude and interpreted using the 3D model SLIMCAT. By late January, there was strong chlorine activation with 33% of the available inorganic chlorine column (HCl + ClONO2) being lost, with amounts of ClO greatly enhanced. By the middle of March, the ClO had been deactivated to ClONO2, which exceeded its original fractional abundance (41%), but HCl stayed low (39%) and the ClO columns were still enhanced. The strong and sustained chlorine activations caused considerable accumulated ozone column loss of (12 ¿ 7)% and (31 ¿ 4)% by the beginning of February and March, respectively. The model showed only (5 ¿ 3)% and (23 ¿ 5)% column loss for the same time periods. By comparison, the 1998/1999 winter measurements showed no significant loss of column ozone but a 10% loss in the model.