The suppression effect of oxygen on Si incorporation in homoepitaxial diamond films grown by microwave assisted chemical vapor deposition is investigated by secondary ion mass spectrometry. The Si depth profile in the multilayered diamond films continuously synthesized with different oxygen addition shows that Si incorporation decreases with increasing oxygen addition into the plasma. A change in the interfacial composition at the quartz glass due to oxygen-promoted surface chemistry may be the origin of this suppression effect. ©1997 American Institute of Physics.