In fast reactor fuel pins with 0.3 m long fuel sections, fission-product cesium has been observed to migrate axially and concentrate at the interface between the uranium-plutonium oxide fuel and the uranium oxide blanket. To aid in the understanding of this behavior, kinetics of the reaction of gaseous cesium with urania have been studied in out-of-pile experiments over the temperature range 600 to 1070 K. The rate of reaction increased with increasing urania temperature, cesium pressure, and OU atom ratio, and is given by: rate = D[PCs(Pa)]0.96±0.13(OU — 2.000) exp[−34 ± 3 kJ/mol)/RT], where D = 9.5 mol Cs m−2 s−1 Pa−1. For in-pile conditions, the rate of reaction of gaseous cesium with a uranium oxide blanket pellet was calculated to be at least 103 times greater than the maximum cesium transport rate, the fissioninduced birthrate of cesium. As a consequence of the rapid kinetics, fission-product cesium is expected to form cesium uranate preferentially in the first several urania blanket pellets, but not in the entire blanket. This is in accord with in-pile data.