In a previous paper (Andreae & Lamb 1951) the ultrasonic absorption in a liquid was deduced thermodynamically, starting from an expansion of the adiabatic compressibility. This procedure has been revised, using a more satisfactory expansion, and a general expression is derived for the absorption due to a single relaxing process. The relaxation time ז
a
of a mechanism
A
, giving rise to a dispersion of the ultrasonic waves in the neighbourhood of a frequency
f
a
, is assumed to be much less than, or much greater than, the relaxation times of all other processes occurring within the liquid. It is then found that processes having a much smaller relaxation time than ז
a
may have an appreciable effect on calculations of the absorption per wave-length due to mechanism
A
. Experimental measurements are presented which delineate a dispersion in carbon disulphide centred around 75 Mc/s at 25° C and 31 Mc/s at -63° C. The dispersion is attributed to a relaxation of the total vibrational specified heat. The absence of any dispersion at low frequencies can be inferred from static compressibility data.