We propose and test an empirical nucleofugality index to rank the leaving group ability of a series of molecular fragments present in nucleophilic substitution reactions of carbonyl and thiocarbonyl derivatives. The nucleofugality index is defined as the group electrophilicity of the leaving group embedded in the substrate that undergoes the nucleophilic attack. The reliability and usefulness of this new reactivity index is tested against experimental kinetic data. (c) 2006 Elsevier B.V. All rights reserved.