We have theoretically studied, in gas phase and using PCM-water solvation, the complexes established by the biologically relevant guanidinium cation and simple aromatic systems (benzene, naphthalene and pyridine). In gas phase only hydrogen bonded complexes were obtained, whereas using PCM-water different cation-pi complexes for the three aromatic systems were found. The interactions established within these complexes have been analyzed by means of the atoms in molecules and natural bond orbital approaches. Finally, experimental evidence of the cation-pi interactions created by guanidinium was found in the crystal structure of N-(5-methylpyridin-2-yl)guanidinium chloride, which was also theoretically analyzed. (C) 2011 Elsevier B. V. All rights reserved.