Infrared spectroscopy and density functional theory calculations have been applied to elucidate the hydrogen bonding interactions between water/dimethyl sulfoxide (DMSO) and a representative pyridinium-based ionic liquid, 1-butylpyridinium tetrafluoroborate ([BuPy][BF4]). It has been found that both solvents can interact with the BuPy+ cation through the aromatic C-H. The strength of the H-bonds involving the aromatic C-H in water is similar to that in pure [BuPy][BF4], but is much stronger in DMSO. For DMSO, when it forms H-bonds with the BuPy+ cation through its S=O group, its back-side methyl groups act as electron donors, while the butyl group of the cation acts as an electron acceptor. For water, when it forms the strong anion-HOH-anion complex, it can also form H-bonds with the aromatic C-H on the BuPy+ cation. This is different from the imidazolium-based ionic liquid, where the strong anion-cation interaction and steric hindrance from the alkyls prevent water molecules from H-bonding with the aromatic C-H other than with the anion.