Results of the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) are compared with vertical ClO profiles measured by the ground-based Millimeter Wave Radiometer MIRA2 inside the vortex during March 1997 at Ny-¿lesund. The influence of the OH+ClO and HO2+ClO reaction branching ratio and of the absorption cross section of Cl2O2 on the calculated mixing ratios of ClO and ozone has been investigated. In the upper stratosphere the ClO mixing ratio is reduced by 90% by using a minor channel of the OH+ClO reaction with a branching ratio of 0.07. A temperature dependent minor channel of the HO2+ClO reaction reduces the upper stratospheric ClO mixing ratio by 22%. Different absorption spectra of Cl2O2 alter the ClO mixing ratios up to 12% at noon at 20 km. This causes differences of 15% in the ozone loss during winter. ¿ 1999 American Geophysical Union