The oxidation of uranium mononitride UN has been investigated and some comparative measurements carried out on uranium monocarbide UC. Powdered UN, of surface area 0.2–0.4 m2/g, oxidizes isothermally at a measurable rate in the temperature range 230–270°C. The reaction kinetics are independent of oxygen pressure (50–760 torr) and the apparent activation energy is 30–32 kcal/mole. Complete oxidation leads to an amorphous product of composition UO3 . N0.2–0.4 which releases nitrogen only with difficulty and recrystallizes to αUO3. The oxidation of UC leads finally to UO3 containing residual CO2. X-ray studies of both powdered and single crystalline UN reveal that U2N3 and UO2 are formed as intermediate oxidation products. These develop as a layered structure, with the U2N3 sandwiched between the underlying UN crystal and an outer layer of UO2. Both U2N3 and UO2 are epitaxially oriented with respect to the UN, the f.c. array of uranium atoms being retained. A novel reaction mechanism is proposed to explain these results. This involves lattice diffusion across the product layers with solid/solid reactions taking place at the UO2/U2N3 and U2N3/UN interfaces. Experiments with single crystals of UC showed epitaxial orientation of the UO2 product, but no evidence for higher carbides.