Trace gas measurements of CO and N2O were performed over Northern Scandinavia during a mountain wave event in the night 1./2. February 1997. The data were taken during the POLSTAR I campaign on board the DLR research aircraft Falcon by the newly developed tunable diode laser absorption spectrometer TRISTAR. Strong tropospheric winds almost perpendicular to the Scandinavian mountain ridge led to the formation of mountain waves. Due to the steep gradient in the CO mixing ratio around the tropopause these mountain waves resulted in extensive modulations in the CO signal. Peak-to-peak amplitudes of up to 70 ppbv and horizontal wavelengths in the order of 10 to 20 km were recorded. Vertical fluxes of CO in the tropopause region are calculated using the CO data and the vertical wind data taken from the standard meteorological equipment of the DLR aircraft. The vertical eddy exchange coefficient shows values comparable to or even greater than those calculated for tropopause folds. We conclude that mountain wave events seem to play an important role as a mechanism for troposphere/stratosphere exchange of chemical constituents. ¿ 1999 American Geophysical Union