Xenotime DyPO4 and GdxDy(1-x)PO4 (x=0.4, 0.5, 0.6) (tetragonal I4(1)amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond anvil cell with insitu Raman spectroscopy. The typical Raman-active modes of the xenotime structure were observed at low pressures and the appearance of new Raman peaks at higher pressures indicated a phase transformation to a lower symmetry structurelikely monoclinic. Raman mode softening was observed, resulting in a line crossing at approximately 7-8GPa for each material and preceding the phase transformation. The onset of phase transformation for DyPO4 occurred at a pressure of 15.3GPa. DyPO4 underwent a reversible phase transformation and returned to the xenotime phase after decompression. The transformation pressures of the solid solutions (GdxDy(1-x)PO4) were in the range 10-12GPa. The GdxDy(1-x)PO4 solid solutions yielded partially reversible phase transformations, retaining some of the high-pressure phase spectrum while reforming xenotime peaks during decompression. The substitution of Gd into DyPO4 decreased the transformation pressure relative to pure DyPO4. The ability to modify the phase transformation pressures of xenotime rare-earth orthophosphates by chemical variations of solid solutions may provide additional methods to improve the performance of ceramic matrix composites.