Abstract The gas phase reaction of chloride ion with alkyl chlorides (methyl, ethyl, propyl, i-propyl, i-butyl, t-butyl and neo-pentyl) has been studied by ab initio methods. Geometries of stationary points along the reaction coordinate have been optimized at the MP2/6-31G* level of theory and improved energies have been calculated with the 6-311 + G(2d) basis. Although all reactions are “narcissistic”, i.e. reactant and product are identical, it is found that some of the transition structures have unequal CCl bond lengths. The breaking/forming CCl bond for the t-butyl system is significantly longer than for the other alkyl groups, for which the change with increasing steric bulk is as expected. The calculated relative activation energies are compared with available gas-phase data and relevant solution values.