The heat capacity of natural xenotime-(Y) and synthetic DyPO4 and ErPO4 crystals was determined by differential scanning calorimetry (DSC) at temperatures of 298.15 K to 868.15 K and a pressure of 0.1 MPa. The aim of the study was to develop a method to accurately measure the isobaric heat capacity (C-P) of rare earth element (REE) phosphates, compare the results to data from adiabatic calorimetric experiments, and evaluate the deviation from ideality of the C-P of the natural xenotime-(Y) solid solution. The measured C-P data (inJ mol(-1) K-1) can be described by the relationships: 185.5 - 751.9T(-0.5) - 3.261e + 06 T-2 for DyPO4; 207.2 - 1661T(-0.5) - 5.289e + 05 T-2 for ErPO4; and 208 - 1241T(-0.5) - 2.493e + 06 T-2 for xenotime-(Y); where T is the temperature in K. The heat capacity data for natural xenotime-(Y) were used to determine the excess function for the solid solution, which yields an excess heat capacity ranging between 7.9 and 10.7%, well within the range of the DSC method used in this study. The experiments indicate that xenotime-(Y) forms a non-ideal solid solution. Future DSC studies will provide important data for developing a solid solution model for the incorporation of REE in xenotime-(Y). (C) 2016 Elsevier B.V. All rights reserved.