Looking for a better knowledge concerning water and ionic liquids (ILs) interactions, a systematic study of the activity coefficients of water in pyridinium, pyrrolidinium and piperidinium-based ILs at 298.2 K is here presented based on water activity measurements. Additionally, the study of the structural effects of the pyridinium-based cation is also pursued. The results show that non-aromatic ILs are interacting more with water than aromatic ones, and among the ortho, meta and para isomers of 1-butyl-methylpyridinium chloride, the ortho position confers a more hydrophilic character to that specific IL. The physicalchemistry of the solutions was interpreted based on dissociation constants, natural bond orbitals and excess enthalpies providing a sound basis for the interpretation of the experimental observations. These results show that hydrogen bonding controls the behavior of these systems, being the anion-water one of the most relevant interactions, but modulated by the anionecation interactions. ; This work is being developed in the scope of the projects CICECO-Aveiro Institute of Materials (Ref. FCT UID /CTM /50011/ 2013), financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement, and LSRE/LCM (Ref. FCT UID/EQU/50020/2013). Imran Khan and Mohamed Taha acknowledge Fundaç~ao para a Ciȇncia e Tecnologia for the postdoctoral grants SFRH/BPD/76850/2011 and SFRH/BPD/78441/2011, respectively. The authors I. Khan also acknowledges for financial support from FCT for the project EXPL/ QEQ-PRS/0224/2013.