Chemical vapor deposited diamond films thermochemically polished on cylindrical steel plates are subjected to Rutherford backscattering spectrometry (RBS) and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDX). A diamond film polished for 25 h at 950degreesC is etched in chromic acid (K2Cr2O7 + H2SO4 + H2O) prior to RBS measurement while two other films also polished at 950degreesC for 8 and 25 h, respectively, are analyzed as polished. The RBS spectra of as-polished diamond films contain peaks of iron contaminant while the surface of the etched film is devoid of iron. Moreover, the concentration/magnitude of the contaminant on the film polished for 25 h is about twofold that of the film polished for 8 h. SEM-EDX spectra taken from dark and bright spots indicate the presence of iron contaminant in as-polished films but its absence in the etched film regardless of the surface topology. Thus, the depth measured by the RBS is assumed to represent the characteristic thickness of the nondiamond carbon layer in which the iron contaminant originating from the polishing plate is embedded. The presence of silicon and oxygen bands in the SEM-EDX spectra of both as-polished and etched diamond films is attributed to their inclusion during film deposition.