Ultrasonic propagation in Al2O3at low temperatures and high frequencies (to 3000 Mc/sec) is discussed in terms of the relations given by Woodruff and Ehrenreich. Phonon relaxation times calculated from thermal conductivity measurements are used. The measured low‐temperature attenuation values at 1315 Mc/sec agree with the calculated values. A comparison of attenuation as a function of frequency is made among Al2O3, Ge, Si, and quartz. On comparing experimental values of attenuation for pure Al2O3and impure Al2O3(ruby) it is found that impurities lead to an increase in the attenuation as would be predicted from the corresponding effect of impurities on the thermal phonon relaxation time.