The reaction mechanism and the development of the aromaticity along the trimerization of acetylene to yield benzene have been analyzed by means of use of the electron localization function (ELF) and the catastrophe theory. The electronic rearrangements associated to bond breaking/forming processes are characterized by four catastrophes, which determine five domains of structural stability of the ELF topology along the intrinsic reaction path. The analysis of the ELF separated into in-plane (sigma) and out-of-plane (pi) contributions shows that the transition structure (TS) has a low 47 electron delocalization, being pi-aromaticity developed at the final stage of the reaction. (c) 2005 Elsevier B.V. All rights reserved.