The first step in gas-phase electrophilic aromatic substitution (EAS) reactions of alkyl carbenium ions with fluoro-, chloro- and bromo-benzene, furan, thiophene and pyrrole, has been investigated by using high-pressure, chemical-ionization and tandem mass spectrometry (MS/MS). Collisionally activated dissociation (CAD) and tandem mass spectrometry, MS/MS, have been utilized for ion-structure determination. The coexistence of σ and π complexes in these reactions is a means of rationalizing the results. It has been established that the extent of π-complex formation decreases with an increased capability of the aromatic to donate electron density. Semi-empirical computations indicate that as π-electron density within the aromatic moiety increases, the energy difference between the more stable σ complex and the π complex increases, which is also consistent with experimental observations. The extent of bonding between the alkyl group and the aromatic of certain π complexes is sufficiently weak that isomerization of the alkyl moiety takes place.