In situ observation by photoelectron spectroscopy using synchrotron radiation was performed to investigate the species and coverage of surface adsorbates while exposing the high-temperature Si(001) surface to SiH2Cl2 atmosphere. Photoelectron spectra of valence bands made it possible to evaluate the clean surface area with dimer dangling bonds (Si-db). Curve-fitting analysis of Si 2p spectra showed that the chlorine adsorption species is only monochloride (Si-CI) in the whole temperature region from room temperature to similar to 700 degrees C examined. From comparison between the surface coverages of Si-db and Si-CI in connection with the 2 x 1 structure of the SiH2Cl2-adsorbed Si(001) surface as observed by reflection high energy electron diffraction, we assigned the hydrogen adsorption state to be dominantly monohydride (Si-H). The temperature dependencies of the surface coverages obtained for Si-CI, Si-H and Si-db are discussed using a surface reaction model of SiH2Cl2 adsorption.