Rate constants and products were measured for over 50 ion-molecule reactions involving SO3 or ions formed from SO3. All negative ions studied reacted with SO3 at or near the collisional rate by a variety of processes including charge transfer, O- transfer, ligand exchange, and clustering. Ambient negative ions tended to form SO-4 or SO-4 core ions. A chemical ionization detection scheme for monitoring SO3 in the atmosphere is presented based on CO-3(H2O)n=0-1 or NO-3(H2O)n=0-1 as the primary ions. The proposed scheme can be used in conjunction with previous schemes for monitoring SO2 and H2SO4 concentrations to measure all three gases simultaneously. The measurements also confirm that the ion chemistry used in separate studies to monitor the reaction of SO3 with H2O is correct and rapid and that the reaction of SO3 with positive ion water clusters does not contribute to H2SO4 production in the atmosphere. ¿ American Geophysical Union 1995