The distribution of the actinides and the composition of metallic fission-product inclusions and oxide inclusions in a mixed-nitride fuel irradiated to 1.29% FIMA in a fast flux has been investigated using an electronprobe microanalyser. Plutonium enrichment was found at open and healed cracks, and a significant decrease in plutonium concentration was measured in the region of the central void. Metallic inclusions were detected in a band at a radial position r/r0 -0.7 to 0.8. The inclusions were of two types: intermetallic compounds of the form UMe3, where Me =ruthenium, rhodium, and palladium, and molybdenum-technetium alloys. Plutonium was absent in all inclusions examined. Oxide inclusions were observed in the outer part of the fuel at r/r0 > 0.5. These were also of two types. The first contained ∼5 wt% plutonium and was situated in the interior of the grains. The second had a plutonium concentration of ∼18 wt% and was found at grain boundaries. The plutonium concentration of both types of inclusions was lower than the nominal concentration of the fuel matrix. During irradiation oxide inclusions were formed in the outer part of the fuel by the oxidation of sesquinitride precipitates. The limit of plutonium solubility in uranium sesquinitride is proposed to be ∼5 wt%.