We present ground based millimeter-wave measurements of Arctic stratospheric ozone at Ny-¿lesund, Spitsbergen (79N, 12E), for winter 1997/98. Vortex averaged chemical ozone depletion rates around the 475 K isentropic level were derived from our observations using measured ozone trends for 20 day periods and calculations of diabatic descent inside the vortex. Back trajectory calculations for air-parcels crossing Ny-¿lesund show that measurements at Ny-¿lesund were representative for vortex air-masses. Significant chemical ozone depletion was observed during one period at the end of February 1998 with loss rates of 32 ppb/day around the 475 K isentropic level. During this period the loss rate was higher than in February 1997. Due to the early breakup of the polar vortex, the chemical ozone depletion stopped in March and the accumulated loss is found to be 0.64¿0.2 ppm in 1997/98. ¿ 1999 American Geophysical Union