The reaction mechanism of the mono-substituted acetylenes trimerization has been analysed by means of the electron localization function ( ELF). The trimerization reactions were characterized in six domains of structural stability of ELF along the IRC pathway. Substituent effects are shown to be varied and important along the IRC. Among the substituent considered in this study ( F, CN, COH and OH), Formyl, the most relevant, promotes reactant stabilization by hydrogen interaction, decreases close shell interactions, increases the distortion energy in the first step, and provides the highest stabilization by aromaticity. (C) 2008 Elsevier B. V. All rights reserved.