In situ aircraft measurements of O3, CO, HNO3, and aerosol particles are presented, performed over the North Sea region in the summer lower stratosphere during the STREAM II campaign (Sratosphere Troposphere Experiments by Aircraft Measurements) in July 1994. Occasionally, high CO concentrations of 200-300 pbbv were measured in the lowermost stratosphere, together with relatively high HNO3 concentrations up to 1.6 ppbv. The particle number concentration (at standard pressure and temperature) between 0.018-1 μm decreased across the tropopause, from > 1000 cm-3 in the upper troposphere to < 500 cm-3 in the lowermost stratosphere. Since the CO sources are found in the troposphere, the elevated CO mixing ratios are attributed to mixing of polluted tropospheric air into the lowermost extratropical stratosphere. Further we have used a chemical model to illustrate that nitrogen oxide reservoir species (mainly HNO3) determine the availability of NO(x) (= NO + NO2) and therefore largely control the total net O3 production in the lower kilometers of the stratosphere. Model simulations, applying additional NO(x) perturbations from aircraft, show that the O3 production efficiency of NO(x) is smaller than previously assumed, under conditions with relatively high HNO3 mixing ratios, as observed during STREAM II. The model simulations further suggest a relatively high O3 production efficiency from CO oxidation, asia result of the relatively high ambient HNO3 and NO(x) concentrations, implying that upward transport of CO rich air enhances O3 production in the lowermost stratosphere. Analysis of the measurements and the model calculations suggest that the lowermost stratosphere is a transition region in which the chemistry deviates from both the upper troposphere and lower stratosphere.