Abstract: Despite being intense in the gas phase, the strength of the interaction between a cation and an aromatic cloud decreases significantly when solvated. This effect has an important impact on a variety of systems where the cation•••pi interaction is known to play a key role. In the case of cation•••pi contacts involving aromatic amino acid residues, the degree of exposure to the solvent can be widely variable, so the interaction can be modulated by the environment in an almost continuous way. Most theoretical studies regarding cation•••amino acid interactions have been performed by considering simple alkaline cations in the gas phase. However, interactions can also be established with more complex cations from other amino acids, such as it is the case with arginine cationic side chain and the aromatic units in phenylalanine, tyrosine and tryptophan. In the present work ab initio and DFT methods were employed to study complexes formed by phenylalanine and a guanidinium cation representing the cationic chain of arginine. The results help understanding the impact of the interaction on the structure of phenylalanine, and constitute a first step towards the study of the effects caused by water molecules in the characteristics of these systems.