Abstract: Biodiesel fuel exhibits similar chemical and thermodynamic properties to that of petroleum diesel fuel and hence it is of particular interest to study the molecular association in biodiesel+diesel mixtures. In this regard, the values of sound velocity, density and viscosity were measured at 303 K in the system of biodiesel (sesame oil methyl ester) and diesel. Some of the acoustical parameters such as adiabatic compressibility, free length, free volume and internal pressure were calculated and are interpreted in terms of molecular interaction between the components of the mixtures. The deviations of the sound velocity from that of experimental values were obtained and the collision factor theory was found to be the best suited. The presence of strong interactions was confirmed in the binary system.