This paper reports on the gas phase reactions of HNO3 with Cl-, Cl-H2O, and Cl-HCl, of Cl2 with Cl-H2O and Cl-HCl, and of HCl with Cl-H2O. Rate constants and product ions have been determined at room temperature and at low pressures (0.7-1.7 mb) by means of a flow tube reactor coupled to a quadrupole mass spectrometer. Apart from Cl-HCl + Cl2, all reactions proceed at the collision rate. The reaction mechanism for all reactions, except for Cl- + HNO3, seems to be ligand switching. The implication of the results on a chemical ionisation method for the in situ derivation of stratospheric nitric acid concentrations is discussed. (Int J Mass Spectrom 181 (1998) 113-121)