Negative ion composition measurements and inferred gaseous sulfuric acid concentrations were for the first time obtained in the winter arctic vortex. The observations were made by a balloon-borne quadrupole mass spectrometer, between 24 and 30 km altitude, on 18 January 1992. The H2SO4 data provide strong evidence for an additional OH source, other than photochemical OH-production from O(1D), operative in the winter arctic stratosphere. The additional OH source might be due to ion-molecule reactions involving ambient ions, formed by galactic cosmic rays. This mechanism seems to explain the presence of gaseous sulfuric acid in the winter arctic vortex and may even influence H2SO4 abundances at mid-latitudes during nighttime. ¿ American Geophysical Union 1994