Analysis of the optical properties of Fe/Si multilayers with a strong antiferromagnetic coupling shows that a metallic FeSi silicide compound with a CsCl type of structure is spontaneously formed at interfaces between Fe and Si during deposition. However, for the Fe/Si system with ultrathin Fe and Si sublayers (less than 10 Angstrom), our optical data show that the resulting structure is close to an amorphous, semiconducting epsilon -FeSi (and may be to beta -FeSi2).