The steam oxidation of Cr-doped UN fuel pellets is analysed during sequential isothermal holds up to 720 degrees C. In situ neutron diffraction results show how Cr is accommodated in a secondary U2CrN3 phase, leading to the formation of a duplex UN/U2CrN3 microstructure. Under corrosion, the oxidation of the two phases begins at 400 degrees C for UN and 430 degrees C for U2CrN3, respectively. Because the UN phase is preferentially oxidised in the presence of U2CrN3, addition of Cr in UN based nuclear fuel is found to accelerate the corrosion rate. At 430 degrees C the oxidation of UN in the UN/U2CrN3 microstructure is similar to 5 times faster than pure UN, increasing to similar to 19 times faster at 460 degrees C. The oxidation of U2CrN3 produces UO2 via the formation of two transient intermediate phases. In situ neutron diffraction enables oxidation processes of UN and U2CrN3 components to be followed separately within the two-phase system.