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The composition of the mantle transition region, characterized by anomalous seismic-wave velocity and density changes at depths of ∼400 to 700 km, has remained controversial. Some have proposed that the mantle transition region has an olivine-rich ‘pyrolite’ composition^,, whereas others have inferred that it is characterized by pyroxene- and garnet-rich compositions (‘piclogite’), because the sound velocities in pyrolite estimated from laboratory data are substantially higher than those seismologically observed^,,. Although the velocities of the olivine polymorphs at these pressures (wadsleyite and ringwoodite) have been well documented, those of majorite (another significant high-pressure phase in the mantle transition region) with realistic mantle compositions have never been measured. Here we use combined in situ X-ray and ultrasonic measurements under the pressure and temperature conditions of the …