X-ray powder diffraction at room temperature and drop calorimetry at T = 1005 K were performed on LnPO(4)-CaTh(PO4)(2) (Ln = La, Ce) solid solutions. The results show excess molar volume and excess enthalpy indicating deviations from ideal behaviour that have been interpreted in terms of lattice strains resulting from the ion size effects of substitution of the Ln(3+) with (Ca2+ + Th4+). (C) 2008 Elsevier Ltd. All rights reserved.