The high pressure-temperature behavior of gallium sesquioxide (gallia) has been investigated by combining first-principles density functional computations and high-pressure x-ray diffraction measurements up to 2 Mbar. Calculations show that at 300 K, the transformation from corundum-type α- Ga2 O3 to Rh2 O3 (II) -type structure at 32 GPa is followed by second one to the Cmcm structure at 141 GPa. This post- Rh2 O3 (II) transition was successfully confirmed experimentally at 164 GPa and 1300±500 K. Quasiharmonic free energy calculations show that both transformations display negative Clapeyron slope. Structural sequences, transition pressures, and Clapeyron slopes are found well comparable in gallia and alumina. Results are suggestive that the high-pressure stability of the Cmcm structure is a general property even in group IIIB sesquioxides in the pressure range beyond 100 GPa, though the structure has cation sites with two distinct oxygen coordination numbers. © 2007 The American Physical Society.