In the present study, both scanning tunneling microscopy (STM) and high-resolution synchrotron photoemission spectroscopy (PES) have been employed to investigate structural transformation and interfacial reactions of barium fluoride (BaF2) deposited on the clean Si(5 5 12)-2×1 surface as a function of the deposited amount of BaF2 as well as the substrate temperature. One periodicity of the (5 5 12)- 2×1 unit along the [665 ̅] direction composed of one-dimensional (1D) structures such as honeycomb (H) and π-bonded (π) chains and dimer-adatom (D-A) and tetramer (T) rows can be considered as three (337) subunits and one (113) subunit. Whether BaF2 molecules are deposited on the clean Si(5 5 12)-2×1 surface held at 900°C or deposited on the clean Si(5 5 12)-2×1 surface held at RT and postannealed at 900°C, it has been observed that the BaF2 molecules are dissociated to Ba and F atoms, and then Ba atoms are adsorbed on the surface and F atoms are desorbed from the surface.When 0.05 Å of BaF2 is deposited on the Si(5 5 12)-2×1 surface held at 900 °C, Ba atoms are adsorbed selectively on H chains or D-A rows. With increasing BaF2 deposition amount, it was found by STM that the transformation from Si(5 5 12) to (337) with (113) facets. When the surface is saturated by the deposition of 0.4 Å of BaF2, only (113) facets develop distinctly. On the (337) facets, Ba atoms are adsorbed on the D-A row of (337) units composed of H chains and D-A rows. On the (113) facets, the periodicity is 2× along the [332 ̅] direction, while the periodicity along the [11 ̅0] direction is converted to 4× or 6×, with increasing BaF2 deposition amount. On the contrary, when 0.4 Å of BaF2 is deposited on the Si(5 5 12)-2×1 surface at RT and postannealed at 900 °C, it was been detected by STM that the surface is mostly occupied by (337) facets saturated by Ba atoms. However, the Ba-Si bond-related component of the Ba 4d and Si 2p core-level spectra says that the bonding environment between Ba and Si is identical regardless of (337) facets or (113) facets.