The heat capacity of thorium dioxide was determined from 10 to 305K in an adiabatic calorimeter. As expected, no anomaly was observed in the heat capacity.

The heat capacity of a two-gram sample of neptunium dioxide was determined from 10 to 315 degrees K in an adiabatic calorimeter. At 298.16 degrees K, the molal entropy and enthalpy calculated from the heat capacity data are 19.19 (plus or minus) 0.1 cal degree (exp -1) and 2770 (plus or minus) 15 cal, respectively. A pronounced hump was found in the heat capacity curve of neptunium dioxide at 25.3 degrees K similar to that found by Long, Jones, and Gordon in uranium dioxide at 28.6 degrees K. This anomaly in the heat capacity is suggestive of a cooperative phenomenon and is attributed to the incidence of antiferromagnetism below the temperature of the maximum in the heat capacity. A calorimeter for gram-scale samples and a modification of the cryostat design of Ruehrwein and Huffman so as to employ only liquid helium and liquid nitrogen as refrigerants are described.