We have prepared samples of Mg2Si on atomically clean Si(100) surfaces under ultrahigh vacuum conditions and used X-ray photoelectron spectroscopy to study the nature of the chemical bond. Values for the charge transfer between Si and Mg are derived from the XPS spectra and compared with values determined theoretically. An ionicity of 9% was predicted from the theory, which agrees well with the value of 8% derived from the size of the chemical shift associated with the XPS spectra.