To improve the understanding of the molecular interactions of water with tetraalkyl ammonium-based ionic liquids (ILs) such as tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetrabutylammonium hydroxide, thermophysical properties such as density (rho), speed of sound (u), viscosity (eta) and refractive index (n(D)) were measured and a computational study using COSMO-RS was performed. The derived properties such as excess volumes (V-E), deviation in isentropic compressibilities (Delta kappa(s)) deviation in viscosities (Delta eta), and deviation in refractive indices (Delta n(D)) under the same experimental conditions for these systems were also estimated. The observed V-E and Delta kappa(s) values are negative over the entire composition of ILs at all investigated temperatures, whereas Delta eta and Delta n(D) values are positive under the same experimental conditions. These results reveal that the ammonium-based ILs significantly affect the intermolecular interactions between the solvent molecules. The computational study allows a qualitative analysis of the results in terms of the ion dipole interactions, ion-pair formation, and hydrogen bonding between ammonium-based ILs and water.