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Monazite and xenotime are common metamorphic phases that may be reliably U/Th-Pb dated to obtain absolute ages. The utility of these minerals is significantly enhanced if a crystallization thermometer can be developed and applied to better establish the temperature-time (T-t) paths of crustal terranes. Here we report experiments in which we have modeled the T-dependent Rare Earth element (REE) cationic exchange between coexisting monazite and xenotime to derive a new thermometer. We present a thermometer in which phosphates were cocrystallized from 1150 ◦C to 850 ◦C at 1 GPa in a Y-REE-P2O5-NaCl-H2O system, with oxygen fugacity buffered at the Ni-NiO equilibrium. The composition of the phosphates was quantified using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Results reveal strong correlations between log10 (Π XLREE Xnt ×XY Π Mzt XLREE Mzt ×XY Mzt ) (at. %) and 104/T(K) and our preferred calibration is: log (ΠXLREE Xnt × XY Π Mzt XLREE Mzt × XY Mzt ) = ( 􀀀 6714 ± 2264) T (K) 􀀀 (0.79±1.76) where LREE = La, Ce and Pr, α = activity of a cation in a phase, and ΠαY/REE Mzt/Xnt refers to the product of activities of Y and/or REEs in a phosphate phase. The errors are 2 s.e. The greatest strength of this thermometer is its versatility. One can obtain derivative thermometers based on select elements rather than the entire suite of REEs. We showcase our thermometer’s adaptability by applying it to two studies that have published REE data on monazite and xenotime from some quartzo-feldspathic psammites and garnet-bearing pelites that experienced amphibolite facies metamorphism from the Naver nappe in the Northern Highlands Terrane, Scotland. The main calibration shown above, as well as four derivative single-element thermometers (Y, La, Ce and Pr) were applied to the first study. Upon applying these thermometers, we find that the calculated metamorphic Ts agree well with the regional metamorphic facies. Thus, this versatile thermometer can be used in geologic environments where monazite and xenotime co-crystallized.