The high-pressure phase transitions and the equations of state of the barium chalcogenides BaS and BaO were investigated by energy-dispersive x-ray diffraction using the Cornell High Energy Synchrotron Source (CHESS). For BaS, the rocksalt (B1) phase was found to be stable between 0 and 6.5 GPa and the cesium chloride (B2) phase was stable between 6.5 and 89.0 GPa, the highest pressure reached in this experiment. For BaO, three structural phases were found between 0 GPa and the highest pressure of 60.5 GPa. BaO(I) is in the rocksalt phase between 0 and 10.0 GPa, BaO(II) is in the hexagonal NiAs phase between 10.0 and 15.0 GPa, and BaO(III) is in the tetragonal PH4I phase from 15.0 GPa up to the highest pressure of 60.5 GPa. From the variation in the intensities of the diffraction lines for BaO(III) with pressure, we conclude that this tetragonal PH4I phase is continuously approaching a cesium-chloride-type structure as the pressure increases. This behavior of BaO(III) is a new phenomenon in the study of barium chalcogenides and alkaline-earth oxides. The potential for metallization of BaS and BaO under high pressure is also discussed. © 1986 The American Physical Society.