The thermodynamic performance of coolant gases is compared for given pressures and temperatures and for the same maximum surface temperature of the fuel element. Two relationships allow comparison of power output and channel dimensions for different gases and various operating conditions such as fixed ratio of pumping power-to-thermal power. Except for steam, the relative merit of gases is found to depend more on temperature than on pressure. From a thermal viewpoint, the conditions under which coolants are compared may alter the order of preference of gases such as helium or carbon dioxide.