This is a study of a potential low viscous protic IL to be used in CO2 capture processes. High-pressure phase equilibria data were measured for diethylammonium butanoate [DEA][Bu] + CO2, CH4 or N-2 from 303 to 333 K and up to 20 MPa. The experimental data of density, viscosity and sound speed were measured from mixtures of [DEA][Bu] + water with different water amounts and at several temperatures. This protic ionic liquid was characterized by water content, FT-IR, H-1 and C-13 NMR, which were used to determine the IL molecular structure and estimate its purity, before and after phase equilibria experiments. A comparison among 38 ionic liquids indicates that [DEA][Bu] has a significantly low viscosity. Phase equilibria data were adjusted by Peng-Robinson and sPC-SAFT EoSs with high accuracy, mainly for CO2 data. sPC-SAFT also well fitted the density and sound speed of pure [DEA][Bu] (dev. < 0.2%). [DEA][Bu] Henry's law constants and ideal CO2/CH4 selectivity values were compared with those of other ILs at different temperatures, molecular weights and pressures. The comparisons indicate that [DEA][Bu] presents moderate to low CO2 solubilities and CO2/CH4 selectivities but a low pressure sensibility. Thus, at high pressures, this PIL would present higher selectivities than most ILs studied. (C) 2017 Elsevier B.V. All rights reserved.