Uranium diboride (UB 2 ) was produced via arc melting and mixed with U 3 Si 2 material via coldpress and sintering. Composite materials, from 0-100% UB 2 were placed in high temperature flowing steam and mass gain observed. It was seen that the addition of 10 wt% UB 2 to U 3 Si 2 increased the onset of reaction by 100 • C compared to pure U 3 Si 2 in the same conditions. 50 wt% UB 2 /U 3 Si 2 pellets maintained this increase, alongside a significant reduction in the ejection of material typically seen in U 3 Si 2 -steam reactions. From these results, a mechanism for both composite reaction and the performance of U 3 Si 2 in steam is proposed.