The behavior of PdO at high pressure was studied by energy-dispersive x-ray diffraction of samples contained in diamond anvil cells. PdO undergoes a first-order transition at about 12 GPa. The new phase is tetragonal and of similar cell dimensions to the low-pressure phase. However, it is more compressible along c and much harder along a. The volume change is 1.7%. It is likely that the new phase has the rocksalt structure, tetragonally elongated due to the low-spin d8 electron configuration of palladium (II). The zero-pressure cell parameters and bulk moduli are (low pressure phase) a=3.042(1), c=5.351(3), K=28052 GPa; (high pressure phase) a=2.982, c=5.383, K=54520 GPa. One sample prepared was found to be a mixture of PdO with a cubic material [Fm3m, a=4.043(5) at ambient], deduced to be a suboxide PdOx with x approximately 0.210.27. Under pressure, the two phases reacted reversibly: PdO was consumed and x increased with increasing pressure. This led to expansion of the suboxide unit cell with pressure up to 1.73 GPa, above which pressure the calculated value of x remained approximately constant and the lattice constant decreased in the usual fashion. It is evidently possible to construct mixtures of these two phases which show zero total change in the lattice constant of the cubic phase over a selected range of pressure.