Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)This study reports the synthesis of a new protic ionic liquid, bis(2-hydroxyethyl)ammonium butyrate (2-HE(2)AB), performed by a Bronsted acid-base reaction between butanoic acid and bis(2-hydroxyethyl)ammonia. The new ionic liquid was characterized by 1D hydrogen NMR spectrum. The ionic liquid shows complete solubility in water, methanol, and ethanol, and is partially soluble in methyl acetate, ethyl acetate, and propyl acetate, while it is not soluble in some alkanes. Density, refractive index, and vapor-liquid equilibrium were measured for the binary system 2-HE(2)AB + methyl acetate at atmospheric pressure. Furthermore, density, refractive index, and liquid-liquid equilibrium were measured for the binary systems 2-HE(2)AB + ester (methyl acetate, ethyl acetate, or propyl acetate) at 293.2 K. The Peng-Robinson equation of state, coupled with the Wong-Sandler mixing rule, was used in the thermodynamic modeling of density, vapor-liquid, and liquid-liquid equilibrium data. The COSMO-SAC activity coefficient model was used to calculate the activity coefficient within the Wong-Sandler mixing rule. The calculations show deviations for density, for the vapor-liquid equilibrium, and for the non-polar and polar phases of the liquid-liquid equilibrium within 13.0, 0.1, 132.5, and 23.8 %, respectively.19912631269Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)