Zenith sky measurements of scattered light have been made at Ny-¿lesund, Spitsbergen (79 ¿N, 12 ¿E) since the beginning of 1995. A differential optical absorption spectroscopy (DOAS) algorithm has been used to retrieve differential slant column densities (DSCD) of IO for the observations from 1995--1998. IO was detected every year for several days within each month. Model simulations result in an estimated stratospheric IO mixing ratio in the range from 0.65 to 0.80 (¿0.2) pptv in polar spring 1997. In addition, indications for the presence of tropospheric IO have been found in late spring and summer of each year. ¿ 2000 American Geophysical Union