As a step in developing new standards of heat capacity applicable up to very high temperatures, the heat content (enthalpy) of thorium dioxide, ThO2, relative to 273 °K, was accurately measured at ten temperatures from 323 to 1,173 °K. A Bunsen ice calorimeter and a drop method were used to make the measurements on two samples of widely different bulk densities. The corresponding heat-capacity values for the higher density sample are represented within their uncertainty (estimated to be ±0.3 to 0.5%) by the following empirical equation (cal mole-1 deg-1 at T °K): At 298 °K this equation agrees with previously reported low-temperature measurements made with an adiabatic calorimeter. Values of heat content, heat capacity, entropy, and Gibb's free energy function are tabulated from 298.15 to 1,200 °K.