A systematic study of a series of room-temperature ionic liquids, belonging to the alkylammonium nitrate family (XAN), was carried out at 298.15 K and 0.1 MPa with the aim of investigating the effect of the cationic chain length on some thermo-physical properties and their behavior in the organic solvent N -methyl-2-pyrrolidone (NMP), over the whole concentration range. Experimental densities were used to calculate molar volumes, V m, and excess molar volumes, V E. Complementary information was obtained by isothermal titration calorimetry that provided the values of the heat of mixing, H E, and the excess partial molar enthalpies of each component, H¯1E and H¯2E. The density values of neat XAN samples decreased as the alkyl chain length of the cation increases, whereas, the addition of the methoxy group to the considered smaller alkyl chain resulted in an increase of density. Negative VE and HE values were found for each XAN + NMP system, indicating the presence of strong attracting interactions between the constituents.