Enthalpy of formation (Delta H degrees(f)) of single phase orthophosphate solid solutions La(1-x)Ln(x)PO(4) (Ln = Eu, Gd; 0 < x < 1) with monazite structure has been determined by high temperature oxide melt solution calorimetry. The results show a non-ideal behavior of the excess enthalpy of mixing of the solid solutions. This effect is more pronounced for the La(1-x)Ln(x)PO(4) solid solution series and can be attributed to the larger ion size mismatch between La and Gd. The resulting excess enthalpies of mixing given by the Margules interaction parameter of 2.5 +/- 2.6 kJ.mol(-1) and 11.4 +/- 3.1 kJ.mol(-1) for La:Eu and La:Gd cases respectively are in good agreement with data from previous modelling studies. The data are consistent with the formation of thermodynamically stable solid solution in the entire compositional range as is seen by the XRD and Raman measurements. These results are essential for validation of the thermodynamic models that are applied for characterization of the thermodynamic parameters and for the assessment of the long-term stability of monazite solid solution matrices as ceramics for immobilization of radionuclides for nuclear waste disposal. (C) 2016 Elsevier Ltd.