We have theoretically studied, using PCM–water solvation, the cation–π and π–π complexes established by the biologically relevant 5-substituted 2-pyridinylguanidinium derivatives and electron-rich and electron-depleted aromatic systems (benzene and (hexafluoro)benzene). In condensed phase using PCM–water and M06-2X/6-311++G(d,p) different cation–π and π–π complexes were found. The interactions established within these complexes have been analyzed by means of the Atoms in Molecules and Natural Bond Orbital approaches and electron density difference maps have been calculated. Finally, the effect of the cation–π and π–π complexation on the aromaticity of pyridine, benzene and hexafluorobenzene was evaluated by calculating the corresponding aromaticity indexes, NICS0, 1 and 2 as well as the NICS on the 0.001 a.u. isodensity surface.