The chemical composition changes of silicon and phosphorus in the process of native oxide formation of both heavily phosphorus-doped Si(1 0 0) and polycrystalline Si prepared by BF-treatment were monitored for a period of 1 year, using angle dependent X-ray photoelectron spectroscopy (XPS). Right after HF-treatment, significant amounts of unoxidized-P (P*) and a new chemical state of Si (Si*) having an unstable bond were detected in the surface region of heavily P-doped Si. With decreasing depth, the concentration of P* increased, whereas the Si* decreased. The surface segregated P* was larger for P-doped Si(1 0 0) than for P-doped poly-Si. During the growth of native oxide, the distributions of P* and Si* in the underlying Si surface did not change largely with oxide thickness. A part of P atoms also reacted with oxygen and incorporated into Si-O-P network in the native oxide, and the amount of P-oxide increased with the progress of oxidation. The decomposed Si 2p spectra of growing oxide films on both heavily P-doped Si showed that the dominant component was not the Si4+, but Si3+ state until the oxide thickness went up to over about 1.5 nm, while that for the low P-content Si was Si4+, except at the very narrow interface.