Measurements have been made of the velocity of propagation and the absorption of ultrasonic waves in liquid triethylamine over the temperature range 25 to 70 degrees C and at frequencies of approximately 23, 66, 107, 148 and 192 Mc/s. The absorption results demonstrate the existence of a relaxation process which is attributed to the perturbation by the sound wave of an equilibrium between rotational isomers. The activation energy barrier and the difference in energy between the states are evaluated from an analysis of the results. The calculated dispersion of velocity is less than 1%. Values of the specific heat of triethylamine are required in the analysis and are given in the paper for the temperature range 25 to 60 degrees C. The results are discussed in the light of other measurements concerned with rotational isomers.