Deposition of thin diamond films on silicon (Si) substrates and in a reactor with fused silica walls can lead to the incorporation of Si impurities. In the present work, impurities in the bulk of the films were analyzed quantitatively using complementary diagnostic techniques (elastic recoil detection, electron microprobe analysis and secondary ion mass spectrometry), while surface analysis was achieved with x‐ray photoelectron spectroscopy. The Si contamination level in the bulk reaches up to 0.16 at. %. We show that the presence of Si impurities correlates with the fluorescence background that accompanies the 1332 cm−1diamond peak in the Raman spectra. Experiments were performed to distinguish between the Si originating from the wall and from the Si substrate. The effect of O2added to the process gases is also investigated. The diamond films were prepared in a recently developed plasma reactor using a novel configuration of surface‐wave‐sustained discharge: the reactor operation is akin to that of the well‐known plasma‐ball systems. ©1996 American Institute of Physics.