Abstract: Interactions between cations and aromatic molecules can be often find on a large number of biomolecular systems, such as proteins and receptor-ligand complexes. In addition, its important role in biological processes such as molecular recognition, drug action, and protein folding has been revealed by extensive experimental and theoretical investigations. Most previous theoretical investigations of cation···p interactions, have been focused on their characteristics in the gas phase, with less attention to their behavior in aqueous environment. In this latter case, most of the work has been focused on the whole effect of water solution as a medium on the cation···p interactions. Hence, it is important studying the influence of individual water molecules on the interaction as the cation···p complex is sequentially hydrated. The ammonium···phenol and methylammonium···phenol cation···p complex was selected as a model system to explore how water molecules affect the cation···p interaction. To mimic the process of water molecules binding to a cation···p complex, water molecules were introduced into the complex one by one. The purpose of this study is studying the geometrical characteristic of cation···p complexes with different numbers of water molecules and to investigate how the binding of water molecules to an existing cation···p complex affects the cation···p interaction.