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Frequency and time domain analysis of amplitudes of long‐period (∼20 s) P and S waves diffracted around the earth's core and registered at the WWNSS and CSN stations show that the velocities at the base of the mantle have slightly positive gradients. This result substantiates the previous ray parameter study of Mula and Müller (1980). Such velocity gradients at the base of the mantle imply that there is no significant departure from homogeneity and/or adiabaticity of the base of the mantle from the rest of the lower mantle as suggested by, for example, Bolt (1972), Jones (1977), Jeanloz and Richter (1979), and Doornbos and Mondt (1979b). In addition to this the Qα factor for the base of the mantle is found not to be less than 250.