AbstractThe rate of the reaction between ethyl tosylate and chloride as well as bromide ion has been measured in hexamethylphosphoric triamide in the presence of several counterions. Rate constants for the free ions at 25.0° are 14.4 mole−1 for chloride and 2.0 mole−1min−1 for bromide, both independent of the cation used. The equilibrium constants for ion pair dissociation of lithium‐, ammonium‐ and tetra‐n‐butylammonium chloride are 3.97 × 10−2, 1.52 × 10−3 and 6.36 × 10−3 mole, and for the corresponding bromides 3.30 × 10−2, 9.65 × 10−3, and 9.62 × 10−3 mole.