The temperature dependence of k∞ for a graphite-moderated ThO2-PuO2-fueled lattice has been measured in the high temperature lattice test reactor. Values of measured at equilibrium temperatures from 20 to 1000°C serve as a benchmark for evaluating computational methods and cross sections used in the design of a plutonium-fueled high temperature gas-cooled reactor (HTGR). The calculated and measured changes of k∞ with temperature are in agreement. However, the magnitudes of calculated values are ∼0.8% higher than the measured values for k∞. When plutonium is used as the initial fissile fuel, the temperature coefficient of reactivity is changed favorably from that of an HTGR lattice fueled initially with 235 U. The reactivity decrease associated with elevating this plutonium-thorium fuel to operating temperature is ∼30% less than that obtained with 235U-Th fuel, so the excess reactivity for which the control system must compensate is reduced with plutonium fueling. Above the operating temperature range, plutonium tends to make the temperature coefficient of the HTGR more negative due to increased neutron capture in the 1.06-eV resonance of 240Pu. This effect will improve the self-limiting characteristics of the reactor during a power excursion.