We report the synthesis and characterisation of a series of ammonium ionic liquids (ILs) containing acetate and sulphate anions with alkyl ammonium cation. To understand the thermophysical properties of these synthesised ILs with polar solvent N-methyl-2-pyrrolidone (NMP), precise measurements such as densities (rho), speed of sound (u), and viscosity (eta) over the entire mole fraction range have been measured at 5 K intervals within the temperature range of (298.15 to 313.15) K under atmospheric pressure. The excess molar volume (V-E), the deviation in isentropic compressibility (Delta kappa(s)), and deviation in viscosity (Delta eta) were predicted using these temperature dependence properties as a function of the concentration of ILs. The Redlich-Kister polynomial was used to correlate the results. The ILs investigated in the present work included diethylammonium acetate (DEAA) [(CH3CH2)(2)NH][CH3COO], triethylammonium acetate (TEAA) [(CH3CH2)(3)NH][CH3COO], diethylammonium hydrogen sulphate (DEAS) [(CH3CH2)(2)NH][HSO4], and triethylammonium hydrogen sulphate (TEAS) [(CH3CH2)(3)NH][HSO4]. The intermolecular interactions and structural effects were analysed on the basis of the measured and the derived properties. The information reported here in terms of ion-dipole interactions, ion-pair interactions and hydrogen bonding between ILs and NMP, elucidates the influences of ions and also the temperature effects on the thermophysical properties. (C) 2012 Elsevier Ltd. All rights reserved.