Sixteen unencapsulated mixed-oxide fuel pins were irradiated in EBR-II to ∼5 at.% burnup at 16 kW/ft nominal peak linear power. The fuel fabrication parameters were varied within the ranges proposed for commercial liquid-metal fast breeder reactors (LMFBR) except that pin diameter was ∼15% larger than usually specified. No cladding failures occurred. Pin growth was substantially greater in Type 304L stainless-steel-clad fuel pins than in Type 316 clad elements. Fission gas release to the internal void volume ranged from 73 to 97%. Experimental burnup values were 10 to 12% less than calculated values and varied across the subassembly in a manner that indicated a significant fission rate elevation from an adjacent extra-worth driver subassembly. Substantial attack was found in the cladding at its inside surface and the extent of this attack (up to 0.009-in. intergranular penetration at about 1150°F) correlated with temperature but not with any fuel fabrication parameter. The penetrating fission product material contained Cs, Mo, and Te. The unusually large amount of fuel/fission product/cladding reaction may be related to the high carbon content in the mixed oxides and/or to a short high-temperature operating period.