The topological analysis of the Electron Localization Function (ELF) has been carried out for the hydrogen abstraction pathway between ethynyl radical (HCC.) and ammonia (NH3). The IRC path, minima ((HCCHNH2)-H-..., (HCCHNH2)-N-...) and transition structure have been calculated at the UB3LYP/6-311++G(2d, 2p) computational level. The 'closed-shell' and 'spin-polarized' formula of ELF are used for an analysis. A detailed study of the topology of ELF (the basin population and spin density redistribution) shows that the reaction consists of three main steps distinguished on the IRC path. First, the N-H bond in ammonia is broken, then the hydrogen atom with a population of ca. 0.7 e is transferred and finally the C-H bond in acetylene is formed. It is interesting to note that all chemically important changes occur after the transition state. (c) 2006 Elsevier B.V. All rights reserved.