The enthalpy increments of the (Th1-y, Puy) O-2 solid solution with y = 0.03, 0.08, 0.30, 0.54 and 0.85 and the PuO2 and ThO2 end-members were measured using drop calorimetry in the temperature range 476 to 1790 K. The heat capacity was obtained by derivation of the obtained enthalpy data with respect to temperature. The presented results for PuO2 and ThO2 are compared with the available literature data whereas the results obtained for (Th1-y, Puy) O-2 solid solutions were compared with the Neumann-Kopp's molar additivity rule to search for possible non-ideal behavior. This paper presents the first heat capacity data obtained for this system. (C) 2013 Elsevier Ltd. All rights reserved.