Abstract: Densities and ultrasonic velocity of binary mixtures of acetophenone with benzene at 303.15, 313.15 and 323.15 K were measured over the entire composition range. From these experimental data, the internal pressure (πi), free volume (Vf), Gibb’s free energy (GE), viscous relaxation time (τ) were calculated. Also, the excess internal pressure (πi) and ultrasonic velocity (UE) and excess free volume were calculated. Ultrasonic velocities theoretically evaluated using Nomoto’s relation, Rao’s specific sound velocity relation and Junjie’s equation are compared with experimental values to check applicability of these equations to the systems studied. The relative merits of these theories and relations were discussed.