Ultrasonic absorption and velocity in acetic acid and triethylamine were measured up to pressures of 3280 kg/cm2. The results show that the relaxation frequency in each liquid is independent of pressure to within experimental accuracy (±5%). The pressure data confirm the assumption of Heasall and Lamb that the loss in triethylamine is a rotational isomeric effect. The results in acetic acid are in agreement with the predictions of the monomer‐dimer theory.Comparison with previous results in carbon disulfide and glycerol shows that, depending upon the mechanism, increasing the pressure can cause the relaxation frequency in a liquid either to increase, decrease or remain constant. It appears that the pressure dependence of the relaxation frequency can be useful in distinguishing the type of relaxation present.