Out -of -reactor experiments to determine the compatibility of mixed uranium -plutonium nitrides (UN -20 wt% PUN) and hypostoichiometric mixed oxides (lJ02 -25 wt% Pu02) both helium and sodium bonded to 304-stainless steel cladding have been performed. Capsules were heated for periods as long as SO00 hours at temperatures of 650°C arid 1000°C. No reaction or corrosion layers were observed on adjacent fuel -clad surfaces in cases of both oxide and nitride fuels under the time -temperature conditions investigated.