The surface of polycrystalline diamond films during reactive ion etching in electron cyclotron resonance SF6 and O-2 plasmas has been examined by scanning electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. A SF6 plasma required a high voltage in negative substrate biasing in order to achieve an equivalent etching rate to an O-2 plasma. This was ascribed to the codeposition effects and the low mean energy of ions upon impact in a SF6 plasma. The elemental composition of the amorphous layer formed at the etched film surface in the two plasmas was almost equivalent and mainly composed of carbon and oxygen, except a low concentration (less than or equal to 1%) of sulfur and fluorine in the case of a SF6 plasma. The significant codeposition on diamond surface was the most likely the result of plasma-wall interactions induced by the fragments from SF6, and highly responsible for suppressing the ratio of lateral to vertical etching rate leading to anisotropic etching in a SF6 plasma, in contrast with the little or no codeposition leading to isotropic etching in an O-2 plasma. The results give the substantial implications for the etching process in practical SF6/O-2 plasmas utilized in device fabrication.