New working pairs of room-temperature ionic liquids (RTILs) and water offer an opportunity to replace the highly corrosive and partly immiscible working pair of lithium bromide (LiBr) and water in absorption cycles like the absorption chiller. To estimate the suitability of these working pairs, the knowledge of thermophysical properties is inevitable. Due to the lack of literature data the following properties of two RTIL-water mixtures will be presented in this paper. Vapor-liquid equilibria (VLE) of the binary mixture of water + diethylmethylammonium trifluoromethanesulfonate ([DEMA][OTf]) and the ternary mixture of water + [DEMA][OTf] + diethylmethylammonium methanesulfonate ([DEMA][OMs]) were measured in the temperature range T = (293.15 to 353.15) K. The VLE measurements were carried out by Fourier transform infrared (FTIR) spectroscopy in a dynamic cell. The experimental VLE data were fitted with the nonrandom two-liquid (NRTL) model. Heat capacities, densities, and viscosities of the binary mixtures water + [DEMA][OTf] and [DEMA][OTf] + [DEMA][OMs] were measured. The measurements of the heat capacity were conducted via differential scanning calorimetry (DSC) in the temperature range T = (293.15 to 363.15) K. The density was measured with a pycnometer and the viscosity with a falling sphere viscometer. The temperature range for both was T = (293.15 to 353.15) K Diffusion coefficients of water in the RTILs were determined by pulsed field gradient-nuclear magnetic resonance spectroscopy (PFG-NMR) in the temperature range of T = (288 to 313) K.