Thermodynamic equilibrium calculations have shown that, at the elevated temperatures anticipated during se-vere accidents, uranium oxide-boron carbide interactions are likely to result in significant boron volatilization and the formation of uranium borides. This interaction is further investigated through transient experiments whereby boron carbide pellets are submerged in uranium dioxide melts held at elevated temperatures beneath inert atmospheres. X-ray diffraction and electron-microscopy analyses revealed the aerosols generated by the interaction to be rich in boron sesquioxide (B2O3) and uranium oxide. The solidified melt ingots recovered post-test exhibited highly heterogeneous chemistry with some samples richer in UO2 and others richer in U3O8. Thermodynamic equilibrium calculations indicate that this could be explained by heterogeneity in redox envi-ronment. The second, larger-scale, interaction, with an increased boron inventory, indicated uranium oxide -boron carbide reaction formed either uranium boride or uranium borocarbide.