The rate constants for several ion-molecule reactions involving chlorine-containing ions or molecules have been measured at 300 K with a flowing afterglow apparatus. These include the reaction of O- with CCl 4, several negative ions with HCl, ClO- with several neutrals, and Cl- with O3. These reactions are used to place limits on the electron affinity of ClO: E.A.(ClO)=1.95±0.25 eV. The implications of these results to the negative ion chemistry of the atmosphere are discussed.