To obtain information the characteristics of relaxation phenomena in liquids, measurements of ultrasonic absorption were performed in seven binary mixtures having nitrobenzene as one component.The absorption coefficients in some systems (benzene, chloroform) confirm the results obtained previously in mixtures formed by an unassociated, very absorbing liquid and another component with much smaller absorption coefficient. In this case, the absorption coefficient decreases quickly when small quantities of the second liquid are added to the first one. The explanation of this behavior seems to be found in the fact that an increase of the efficiency of collisions between the molecules occurs. When different molecules are added to the high absorbing liquid, they decrease the time necessary to establish equilibrium among the internal degrees of freedom of the high absorbing molecules.In other mixtures the two components have similar absorption coefficients. These systems are of great interest, because the characteristic behavior of the ultrasonic absorption in the mixture then depends on the nature of both pure liquids. If the two liquids are unassociated and their molecules do not interact strongly between themselves, the curve of the absorption coefficientversusconcentration presents a very clear minimum. If, instead, there are strong interactions, the shape of the curve is altered. This would indicate the presence of an additional type of energy loss.In the systems of nitrobenzene and an alcohol, the absorption coefficient, as a function of the mole fraction, has a maximum at an intermediate concentration, just as happens in some mixtures of water and alcohols.