The enthalpy increment of the monazite-type solid solutions of LaPO4 with NdPO4, EuPO4 and GdPO4 has been measured by drop calorimetry at T = 1000 K. The results show deviations (excess enthalpy) from ideal behaviour that have been interpreted in terms of lattice strains resulting from the ion size effects of substitution of La3+ by Ln(3+). For (La0.5Gd)(0.5)PO4 also the temperature dependence has been determined for T= (515 to 1565) K, indicating that the excess enthalpy decreases with increasing temperature. (c) 2006 Elsevier Ltd. All rights reserved.