The chemical composition and surface state of nanopowder particles prepared by milling natural diamond and detonation synthesis were studied by the method of X-ray photoelectron spectroscopy. It has been shown that the primary particles of both nanopowders contain mainly carbon and oxygen atoms. The signals in the photoelectronic spectrum from atoms of nitrogen, sulfur, chlorine and metals do not exceed the noise level. It was found that in primary particles of detonation synthesis nanodiamond and nanopowder obtained by grinding natural diamond, the proportions of carbon atoms in the sp3 hybridization state are ~ 46.5 and ~ 67.8%, sp2 hybridization is ~ 26.8 and ~ 17.4% and in the composition of oxygen-containing functional groups ~ 26.7 and ~ 14.7%, respectively.