We report the results of a H-1 NMR spectroscopic study of cation dynamics in the RT1L, 1-butyl-3-methylimidazolium bromide, in its crystalline state with two different but considerably low water contents, 30 ppm and 200 ppm. The results indicate the presence of a minor liquid-like (or solution) component in the crystalline state in samples with the higher water contents. The relative fraction of this "liquid-like"phase in the crystal is estimated to be ca. 1% around room temperature and increases with increasing temperature up to the " melting (liquidus) point, the behavior of which is well explained in terms of a two component phase diagram. It is also indicated based on the lever rule that H2O molecules exist at substantially high concentration in the liquid-like phase compared to in the supercooled liquid state, which can significantly change both microscopic and macroscopic properties of the RTIL H-1 NMR spectral line width and spin lattice relaxation time measurements indicate that the cations in the liquid-like phase are characterized by a rotational dynamics that is distinct from that of the supercooled liquid state with significantly higher mobility. Such high cation mobility results from the interaction between the cations and H2O molecules in the liquid-like phase that reduces the Coulombic interaction between the cations and anions and decreases local.viscosity. (C) 2014 Elsevier B.V. All rights reserved.