The microstructure of sintered (U,Pu)O2 microspheres and pellets prepared by the sol-gel process was characterized using light and electron microscopy, electron microprobe analysis, and alpha autoradiography. The material is typically homogeneous in composition, and the grain size is usually between 1 and 5 μm. Coarse microspheres have densities >95% of theoretical, but the fine microspheres approach theoretical density. The microstructure of low density pellets consists of high density and medium density regions in a porous matrix.