Gas-phase ion-molecule reactions involving H2SO4 and a variety of positiive and negative atmospheric ions have been studied by using a flowing afterglow apparatus. These ions include NO3-⋅nHNO3(n=0, 1, 2,) and H3O+⋅nH2O(n=0, 1, 2, 3). All of the negative ions studied react rapidly with H2SO4 to form HSO-4⋅nHNO3. These reactions establish a lower limit for the electron affinity of HSO4:4.5 eV. They also indicate that the terminal negative ions in the upper stratosphere will be cluster ions of the type HSO-4⋅nH2SO4⋅mHNO3, which is an accord with recent in situ observations. On the basis of the present laboratory results and the reported in situ ion composition measurements, the H2SO4 concentration at 35 km is estimated to be 6¿105¿1.6 cm-3. It is found that H2SO4 probably plays very little role in stratospheric positive-ion chemistry, since it does not replace H2O in the H3O+⋅nH2O ion clusters.