Recycling of plutonium in light water reactors in the Federal Republic of Germany began in 1966, and through the subsequent years, has reached a commercial state. Irradiation programs and postirradiation examinations (PIEs) of modern, highly soluble mixed-oxide (MOX) fuel of the ammonium uranyl plutonyl carbonate/optimized co-milling type have been carried out since 1981 to evaluate fuel performance and to verify the data base for design. The results of PIEs on MOX fuel rods with burnups ranging from 6 to 47 G Wd/tonne heavy metal are described. The dimensional behavior of the MOX fuel rods is found to be almost identical to that of UO2 fuel rods. Densification and swelling of MOX fuel are governed by the behavior of the UO2 matrix as well as the porosity that develops in the MOX agglomerates. Little uranium-plutonium interdiffusion occurs in MOX fuel irradiated under normal power reactor conditions, but substantial redistribution is found in the high-temperature region of transient-tested fuel. Fission gas release from the MOX agglomerates occurs via the UO2 matrix, resulting in release behavior similar to that of UO2 fuel. A comparison of the relevant physical properties of UO2 and MOX fuel shows that no distinct difference in the fission product release behavior of defective MOX fuel is to be expected. The available data base does not indicate any MOX-specific characteristic that could limit the burnup potential of this fuel compared with UO2 fuel.