Ionic liquids (ILs) are salts, whose ions are not well organized and have one or more delocalized charges. Besides, at least one ion derives from an organic compound. These characteristics explain why ionic liquids are not solids but rather solvents, most of which are green solvents, even at temperatures below 100 °C and at room temperature (RTIL). These solvents are divided into two groups: aprotic and protic ILs, whose origin comes from a proton transfer from a Brønsted acid to a Brønsted base. Particularly, ionic liquids based on the butylammonium anion have been studied in separation processes and in the treatment of coir fiber for bioethanol generation.Thermodynamic properties have been used in both qualitative and quantitative ways to understand the main chemical, physical, and structural phenomena present in liquid systems. In this work, new data for density, speed of sound, and viscosity of binary mixtures containing water + ionic liquid (IL) n-butylammonium methanoate (N4Met) were determined over the whole composition range, in the temperature range of T = 288.15 - 303.15 K and at atmospheric pressure (p = 92.3 KPa). The ionic liquid N4Met was synthesized in our laboratory by neutralization, dripping methanoic acid on n-butylamine under stirring and controllable conditions, i.e. temperature range between 293.15 – 303.15 K, according to the methodology previously described in the literature. Its characterization by 1H-NMR, 13C-NMR, and FTIR spectroscopy techniques presented a purity of more than 99.7 %. A Karl Fischer titration was used to determine the water content in the IL. The amount was less than 0.3198 % w/w. The N4Met was not dried in a high vacuum, because it generates amide and then its purity decreases. A commercial density and speed of sound analyzer manufactured by Anton Paar (Model DSA 5000) and operating at a frequency of 3 MHz has been used to measure the densities and speeds of sound of the pure components and their solutions. The Peltier device of the equipment made it possible to automatically keep the temperature constant at ±0.01 K. A rotational viscometer/densimeter manufactured by Anton Paar (Model Stabinger SVM 3000) was used to measure the viscosities of pure components and their solutions. In addition to the thermodynamic approach, Nuclear Magnetic Resonance (NMR), 1H NMR, 13C NMR, were investigated to elucidate intermolecular interactions among the species in solutions. An Advance III 600 HD Bruker spectrometer has been used to analyze 1H-NMR (operating at 600 MHz) and 13C-NMR (operating at 150 MHz). Deuterated chloroform (CDCl3) has been used as a solvent with tetramethylsilane (TMS) as standard. The results were discussed in terms of specific interactions between chemical species present in the solution.