Densities, rho, speeds of sound, c, and refractive indices, n(D), have been measured for the systems N,N-dimethylformamide (DMF) + propanone, + 2-butanone, or + 2-pentanone in the temperature range from (293.15 to 303.15) K and at T = 298.15 K for the DMF + 2-heptanone mixture. Due to the high volatility of acetone, the corresponding n(D) measurements were developed at T = (293.15 and 298.15) K. The direct experimental data were used to determine the excess molar volumes, V-m(E), and the excess refractive indices, n(D)(E), at the working temperatures. Values of the excess functions at T = 298.15 K, for the speed of sound, c(E), the isentropic compressibility, kappa(E)(S) and for the excess thermal expansion coefficient, alpha(E)(p), were also calculated. The investigated systems are characterised by strong (amide + ketone) interactions, which become weaker when the alkanone size is increased. This is supported by negative V-m(E) values; by the dependence on temperature and pressure of V-m(E), and by positive P-int(E) (excess internal pressure) values. Analysis of the systems in terms of the Rao's constant indicates that there is no complex formation. In addition, negative V-m(E) values also reveal the existence of structural effects, which largely contribute to the excess molar enthalpy, H-m(E). V-m(E) and H-m(E) values increase with the chain length of the 2-alkanone. It allows conclude that the relative V-m(E) variation with the ketone size is closely related to that of the interactional contribution to this excess function. Molar refraction values, R-m, show that dispersive interactions become more relevant for the systems including longer 2-alkanones. (C) 2016 Elsevier Ltd. All rights reserved.