Measurements of sulfate and nitrate in filter samples made during the Global Atmospheric Sampling Program (GASP) from aircraft flying near the tropopause over a period of 28 months beginning in early 1976 are presented. Sulfate and nitrate mixing ratios show a peak near or just above the tropopause during the winter-spring seasons. No peak is evident for sulfate during the summer-fall seasons. The sulfate mixing ratios show a constant average level in 1977–1978 of 0.12 ± 0.04 ppbm above the tropopause and 0.05 ± 0.04 ppbm below the tropopause with higher levels in the spring. Nitrate levels are similar but exhibit greater variability. Correlations between sulfate and nitrate ( r = 0.78) and between sulfate and nitrate, on ozone ( r = 0.80, 0.78) suggest a predominately stratospheric source for these species.