Room temperature surface tension measurements on the ternary system: nitric acid-sulfuric acid-water are reported. There is a dramatic decrease in the surface tension of sulfuric acid-water solutions as nitric acid is added. This decrease is most pronounced in the stratospherically-relevant range of sulfuric acid concentrations, between 50 and 75 wt% sulfuric acid. The surface tension decrease indicates a change in surface energy and surface composition and/or structure associated with partitioning of molecular HNO3 to the surface. The presence of HNO3 at the surface may be related to the reported changes in heterogeneous reactivity of sulfuric acid solutions when exposed to nitric acid. ¿ 1999 American Geophysical Union