The much discussed and controversial problem of ultrasonic absorption in ethyl and methyl acetates has been further investigated experimentally by the method of optical diffraction in progressive ultrasonic waves. In designing the equipment and throughout the measurement, special cares have been taken for the plane progressive nature of the ultrasonic field in the liquid trough, the elimination of stray light effect so that the receiver (a photomultiplier with preamplifier and a frequency analyzer connected in series) could respond with precision the relative light intensity of the first diffracted maxima at different stations along the ultrasonic beam, etc. Automatic recordings of sound absorption coefficients were made possible by the sound level recorder synchronized with the motion of the liquid trough. Several standard liquids have been tested; the absorption coefficients thus obtained agree well with commonly accepted values. For both acetates, measurements were made in the frequency range of 3-30 Mc. at 20℃ For ethyl acetate the results showed that within the experimental error (which was estimated to be about 5%) the observed absorption curve fits well the theoretical one predicted by the phenomenological relaxation theory and the frequency of relaxation occurs at approximately 12 Mc. Repetitious measurements also showed the lack of dependence of impurities on the relaxation frequency.The relaxation frequency for methyl acetate was found to be about 7 Mc. This once more confirms the theory and thus supports the findings of those who argued that a lower relaxation frequency found by some other investigators was spurious.For the relaxational mechanism, the authors agree to that due to rotational isomerism-first proposed by Karpovich.