Ground-based FTIR-measurements have been performed at the NDSC (Network for Detection of Stratospheric Change) station on Spitsbergen (79 ¿N,12 ¿E). We have investigated several well isolated absorption lines of isotopically heavy O3 in the 10 &mgr;m region that allowed us to quantify their natural abundances in the arctic atmosphere for the period April 1994 to August 1995. On the average, our solar spectra show an excess of 15.4¿0.9% in 16O16O18O and 11.2¿1.4% in 16O18O16O, compared to the natural isotopic ratio in atmospheric oxygen. The lunar spectra recorded during polar night give slightly lower results. The margins of error do not include the isotopic uncertainties of the spectroscopic database used (HITRAN), which might cause a shift by several percent enrichment. ¿ American Geophysical Union 1996