The knowledge of the SO2 mixing ratio at the tropopause and in the lower stratospheric is necessary for understanding the formation and the existence of the stratospheric aerosol layer. A new chemiluminescence technique permitted the analysis of SO2 below 1 ng SO2/ml reaction liquid. The filter sampling technique enables a detection limit of (0.01¿0.001) parts per billion by volume (ppbv) SO2. Comparisons with other methods were performed, and flight application problems are discussed. During 1977 and 1978, several aircraft ascents were performed, reaching up to 5.2 km into the stratosphere. In continental air masses the SO2 mixing ratio decreases strongly within the first 5 km, while in maritime air masses a nearly constant mixing ratio with altitude was observed. A mean value of 0.5 ppbv seems to be typical for layers above 6 km, and SO2 mixing ratios between 0.03 and 0.17 ppbv SO2 were found at the tropopause level, increasing slightly above the tropopause. The experimental results were compared with one-dimensional model calculations.