We present an analysis of trace gas correlations in the lowermost stratosphere. In-situ aircraft measurements of CO, N2O, NOy and O3, obtained during the STREAM 1997 winter campaign, have been used to investigate the role of cross-tropopause mass exchange on tracer-tracer relations. At altitudes several kilometers above the local tropopause, undisturbed stratospheric air was found with NOy/NOy* ratios close to unity, NOy/O3 about 0.003--0.006 and CO mixing ratios as low as 20 ppby (NOy* is a proxy for total reactive nitrogen derived from NOy-N2O relations measured in the stratosphere). Mixing of tropospheric air into the lowermost stratosphere has been identified by enhanced ratios of NOy/NOy* and NOy/O3, and from scatter plots of CO versus O3. The enhanced NOy/O3 ratio in the lowermost stratospheric mixing zone points to a reduced efficiency of O3 formation from aircraft NOx emissions. ¿ 2000 American Geophysical Union