In view of the wide scope of molecular interactions between the highly polar compound of N,N-dimethylformamide (DMF) and ammonium ionic liquids (ILs), we have measured thermophysical properties such as densities (rho) and ultrasonic sound velocities (u) over the whole composition range at temperatures ranging from 25 to 50 degrees C under atmospheric pressure. To gain some insight into the several aggregations of molecular interactions present in these mixed solvents, we predicted the excess molar volume (V(E)) and the deviations in isentropic compressibilities (Delta K(s)) as a function of the concentration of IL. These results are fitted to the Redlich-Kister polynomials. The materials investigated in the present study included the hydroxide series of ammonium ILs of tetramethylammonium hydroxide [(CH(3))(4)N][OH] (TMAH), tetraethylammonium hydroxide [(C(2)H(5))(4)N][OH] (TEAH), and tetrapropylammonium hydroxide [(C(2)H(7))(4)N][OH] (TPAH). The intermolecular interactions and structural effects were analyzed on the basis of the measured and the derived properties. A qualitative analysis of the results is discussed in terms of the ion-dipole and ion-pair interactions, and hydrogen bonding between ILs and DMF molecules and their structural factors.