The contribution to carbon basicity of pi-cation interactions in aromatic systems has been investigated in this work by carrying out ab initio calculations on various cluster models. According to various levels of theory, the mono-, bi-, and tridentate H3O+-benzene complexes present a binding enthalpy in the gas phase of about -28 kcal/mol showing intermolecular contact through unconventional hydrogen bonding between H3O+ and the pi-cloud of benzene. The interaction energies calculated for other cluster models (pyrene-benzene-H3O+, coronene-H3O+, and a C54H18-H3O+ cluster) indicate that the size of the basal plane has a slight influence on the strength of this pi-cation interaction, whereas the presence of the pi-pi contacts reinforces the electrostatic interaction with the H3O+ cation. These theoretical results support experimental data on the ability of the basal planes to contribute to carbon basicity and suggest that pi-cation interactions may play an important role in the surface chemistry of carbon materials.