The topology of the potential energy surface associated with 1-pentene-Cu + and 1-octene-Cu + was investigated through the use of high-level density functional theory calculations. This theoretical survey, together with a combination of tandem mass spectrometry and isotopic labeling experiments carried out for 1-octene, confirm that the pseudo-insertion mechanism tentatively proposed by Fordham et al. [J. Mass Spectrom. 34 (1999) 1007], is the most favorable one. In this mechanism, attachment of the metal cation to the π-system forces a folding of the alkyl chain which favors the formation of an hexa-coordinated intermediate much more stable than the complexes formed along a typical dissociative attachment process. We have also shown that the main features of the pseudo-insertion mechanism do not change when the alkyl chain attached to the C C double bond is much longer. Hence, we can safely conclude from our calculations and the experimental results obtained for 1-octene deuterated derivatives, that this mechanism would explain, in general, the specific reactivity of Cu + with alkenes. We have also shown that the minor loss of H 2 systematically observed in these gas-phase processes, would be related with the ability of Cu + to yield strong agostic interactions either with the methylene groups or with the terminal methyl group of the alkyl chain.