Solid solutions of (Ce,Y)PO 4 were synthesized hydrothermally between 300 degrees C and 1000 degrees C at pressures of 2, 5, 10, and 15 kbar. Experiment products were analyzed by electron microprobe, and their lattice parameters were refined by Rietveld analysis of XRD powder patterns. Over a wide range of bulk compositions, two immiscible phases formed: monazite with a P2 1 /n structure (REEO 9 polyhedron) and xenotime with an I4 1 /amd structure (REEO 8 polyhedron). Variations of the unit-cell dimensions are directly correlated with variations in composition. The boundaries of the asymmetric miscibility gap are strongly dependent on temperature. At 2 kbar and from 300-1000 degrees C, maximum Y concentrations in monazite coexisting with xenotime increase from 3-16 mol%. With increasing pressure, this limb is shifted to higher Y contents so that at 15 kbar it ranges from 6 to about 25 mol% for the same temperature range. In contrast, Ce concentrations in xenotime do not exceed 3 mol% at 1000 degrees C over the entire pressure range. Our experimentally determined boundaries fit surprisingly well with empirical calibrations of the (LREE-HREE,Y) PO 4 - solvus boundaries derived from a suite of low pressure metapelites (3 to 5 kbar) that equilibrated at peak temperatures ranging from 400-700 degrees C. The behavior of the natural (REE,Y)PO 4 system is probably well described by the simple (Ce,Y)PO 4 binary, at least for metapelitic compositions. Many natural samples of monazite coexisting with xenotime show prograde growth zonation with respect to the incorporation of HREE+Y. The combination of our thermometer along with U-Pb and Sm-Nd age determinations of high spatial resolution may thus provide information about prograde branches of metamorphic PTt paths.