The satisfactory irradiation performance of several types of mixed-oxide fuels suitable for the utilization of plutonium in thermal reactors was demonstrated by a significant number of experiments performed in the Plutonium Recycle Test Reactor (PRTR). Heterogeneously and homogeneously enriched packed-particle and pellet mixed-oxide fuels were irradiated at peak linear heat ratings higher, i.e., >655 W/cm (>20 kW/ft), than those employed in the most advanced commercial reactors to significant burnups, i.e., >18 000 MWd/MTM. Early experience in PRTR provided some of the first data related to the effects of hydrogenous impurities on Zircaloy-clad mixed-oxide fuel performance. The demonstrated solution to the problem was achieved through the development of improved analytical and fuel fabrication techniques. Fuel rod cladding becomes oval shaped during irradiation due to creep collapse with the degree of ovality varying among the different fuel types. The fission gas release fraction for vipac mixed-oxide fuels varies linearly from a value of 0.05 for a volumetric average temperature of 600°C to essentially 1.00 for a volumetric average temperature of 2250°C. Homogenization of the PuO2 particles in mechanically mixed UO2-PuO2 commences at fuel temperatures sufficient to cause sintering and equiaxed grain formation during irradiation. PRTR experience indicates that a fuel-cladding reaction that is stoichiometry dependent occurs in Zircaloy-clad mixed-oxide fuels. The satisfactory defect performance of mixed-oxide fuels was demonstrated by several in-service and intentionally defected experiments although there are indications that defect performance may constitute an operating limit at very high linear heat ratings.