The velocity of propagation and the absorption of longitudinal waves has been measured in a hydrocarbon oil for which the shear modulus together with the spectrum of shear relaxation times has been determined previously. A volume viscosity is found which is approximately equal to the shear viscosity and the results are surprisingly similar to those found by Litovitz and his co-workers for associated liquids. Structural relaxation appears to play an equally important part in determining the second viscosity of the hydrocarbon oil as it does in associated liquids. The effects of association are found to give slightly higher values for the limiting moduli at high frequencies (the instantaneous moduli) and are also responsible for the marked temperature dependence of these moduli. In the present case the relaxing part K2 of the modulus of compression and the limiting value of the shear modulus are each sensibly independent of temperature. It also appears that for the liquids studied so far K2≅43⋅G∞. The spectrum of compressional relaxation times is determined and good agreement is found between the measured values of the components of the complex compression modulus and the curves recalculated from the spectrum. The absorption at frequencies above the main part of the dispersion region is governed not by an hysteresis type of loss, as found in associated liquids, but by a residual high-frequency contribution to (α/f2) due to the (unrelaxed) contribution of the vibrational specific heat relaxation.