Discontinuous precipitation (DP) and discontinuous coarsening (DC) reactions have been observed in numerous alloy systems . DP has been observed in the U-Nb system [2, 3, 4, 5]. The U-Nb phase diagram (Fig. 1) exhibits a continuous γ-BCC solid solution at high temperatures and a two-phase mixture of a-orthorhombic and γ-BCC below the 647°C monotectoid isotherm. The DP reaction occurs during continuous cooling and isothermal aging over 300-647°C. No metallographic evidence of a DC reaction in U-Nb has been published, although this is suggested from x-ray observations of distinct changes in the Nb content of the γ phase upon prolonged holding after the DP reaction [2, 3, 6]. This study will provide direct evidence for a DC reaction. Discontinuous and other aging reactions  are undesirable in U-Nb alloys, since they degrade corrosion resistance , ductility , and the shape-memory effect . Hence, an improved understanding of the kinetics of these discontinuous phase transformations in U-Nb alloys is of practical interest.