The thermal reaction of sputter-deposited Ta thin films with polycrystalline CVD-grown diamond substrates at temperatures between 700 degrees C and 1100 degrees C for 1 h is investigated by MeV He backscattering spectrometry, X-ray diffraction, and scanning electron microscopy. The product phases are polycrystalline Ta2C at 900 degrees C and TaC at 1000 degrees C with an initially 135 nm thick Ta film. The first phase formed conforms to the rule of Bene for metal-metal bilayers. No coexistence of Ta2C and TaC is observed during reaction. The second phase formed, TaC, is in thermodynamic equilibrium with carbon.