Enthalpy increments of ThO2, (Th0.9804U0.0196)O2, (Th0.961U0.039)O2, (Th0.941U0.059)O2, (Th0.902U0.098)O2 and simfuelof (Th0.9804U0.0196)O2 were measured using a high temperature Calvet drop calorimeter in the temperature range 375–991 K. The experimental values were used for calculating heat capacities of the compounds, which were compared with Neumann–Kopp’s heat capacity values. On inter comparison of the heat capacity values of solid solution, (ThyU(1-y))O2, with variation in the fraction of ThO2, a trend was observed. In the temperature range of the present experiments, the compound (Th0.961U0.039)O2 showed minimum heat capacity values.