Abstract: The binary system of water and the ionic liquid 1-ethyl-3-methylimidazolium methanesulfonate, [emim][MeSO3], was carefully studied with the initial hypothesis that water might be tightly bound to the ionic liquid up to water mole fractions of 0.5, which would explain why water has been observed to seemingly be deactivated for interfering with water sensitive chemical reactions. Measurement results as a function of composition (generally 0–0.9 water mole fractions) and temperature (generally 40–85°C) were obtained for heat capacity, heats of dissolution, density, viscosity, conductivity, as well as NMR measurements on diffusion, proton chemical shift and T 1 relaxation times of cation, anion and water. The combined results do not confirm our initial hypothesis. An activation energy analysis showed that the same barrier hinders translational motion (self-diffusion) of anion, cation and water, as well as momentum transfer (viscosity) and the water T 1 relaxation. While the activation energy was observed to be linearly dependent on the mol fraction composition, most of the measured physicochemical properties show linear or near linear dependencies to the composition expressed in mass%. A further detailed analysis of the combined experimental data is supportive that the ionic liquid medium remains highly structured even when loaded with water to very high mole fractions. The main structural motif is hypothesized to contain nonpolar domains in close resemblance to micellar aggregation. Thus, the deactivation of the water in chemical reactions may rather be explained by a highly structured ionic liquid framework keeping water physically separated from the reactant. [Copyright &y& Elsevier]