The reactivity of the lithium halides in 83.3% pyridine-dimethylformamide changes from Cl⁻ > Br⁻ > I⁻ to Br⁻ > I⁻ to Br⁻ > I⁻ > Cl⁻ with increasing concentration of the salt from 0 to 0.35 M. This behaviour is explained by ion pairing which reduces the concentration of reactive free nucleophiles. Equilibrium constants K for ion pair dissociation and rate constants k₂ for the reaction of the free nucleophiles were determined from the variation of the observed rate constant with the total halide concentration and from conductivity measurements.