Uranium diboride (UB2) was produced via arc melting and mixed with U3Si2 material via coldpress and sintering. Composite materials, from 0-100% UB2 were placed in high temperature ﬂowing steam and mass gain observed. It was seen that the addition of 10 wt% UB2 to U3Si2 increased the onset of reaction by 100◦C compared to pure U3Si2 in the same conditions. 50 wt%UB2/U3Si2 pelletsmaintainedthisincrease,alongsideasigniﬁcantreductionintheejection of material typically seen in U3Si2-steam reactions. From these results, a mechanism for both composite reaction and the performance of U3Si2 in steam is proposed.