Enthalpy increments of U(1-y)PuyO2 solid solutions with y = 0.45, 0.55 and 0.65 were measured using a high-temperature differential calorimeter by employing the method of inverse drop calorimetry in the temperature range 956-1803 K. From the fit equations for the enthalpy increments, other thermodynamic functions such as heat capacity, entropy and Gibbs energy function have been computed in the temperature range 298-1800 K. The results are presented and compared with the data available in the literature. The results indicate that the enthalpies of U(1-y)PuyO2 solid solutions with y=0.45, 0.55 and 0.65 obey the Neumann-Kopp's molar additivity rule. (C) 2008 Elsevier B.V. All rights reserved.