Auger electron spectroscopy was used to analyze polycrystalline thin-film diamond surfaces following the use of differing methods for the removal of unwanted nondiamond carbon. Exposing the film to a hydrogen plasma at the termination of the growth process is effective for producing a surface that gives an Auger spectrum typical of diamond with little contamination. Strongly oxidizing solutions involving sulfuric acid generate low concentrations of surface sulfur together with an oxide phase. However, in the case of an ammonium persulfate-sulfuric acid etchant solution, the Auger features associated with the diamond more closely resemble those of single crystal material suggesting that this treatment may offer better performance when used during the fabrication of thin-film diamond electronic devices.