Auger electron spectroscopy and temperature programmed desorption studies under ultra-high vacuum conditions demonstrate that even sub-monolayer coverages of oxygen or carbide on polycrystalline Ta significantly degrade the strength of Cu/Ta chemical interactions, and affect the kinetics of Cu diffusion into hulk Ta. On clean Ta, monolayer coverages of Cu will de-wet only above 600 K. A partial monolayer of adsorbed oxygen (3 L O-2 at 300 K) results in a lowering of the de-wetting temperature to 500 K, while saturation oxygen coverage (10 L O-2, 300 K) results in de-wetting at 300 K. Carbide formation also lowers the de-wetting temperature to 300 K. Diffusion of Cu into the Ta substrate at 1100 K occurs only after a 5-min induction period at this temperature. This induction period increases to 10 min for partially oxidized Ta, 15 min for carbidic Ta and 20 min for fully oxidized Ta.