Diamond films have been deposited on titanium substrates by microwave plasma chemical vapour deposition using the bias enhanced nucleation procedure. In the first minutes of exposure to the plasma there is a strong roughening of the titanium surface and a reduction of the oxide layer. Interaction with the gas phase as well as dissolution of the oxygen in the bulk contribute to this reduction. After 30 min plasma treatment the oxygen concentration in the bulk is reduced down to 0.2-0.3