The temperature dependencies of UV-VIS-NIR, IR, and Raman spectra of an MMX-type halogen-bridged mixed-valence complex Pt-2(dta)(4)I (dta = CH3CS2-) and the monovalent dimers Pt-2(dta)(4) and Pt-2(dta)(4)I-2 were investigated. The former undergoes a metal-insulator transition at 300 K, below which a characteristic splitting of C=S stretching mode was observed. The valence localization and its ordering are considered to occur coupled to the metal-insulator transition; -Pt2.5+-Pt2.5+-I-Pt2.5+-Pt2.5+-I- is for the metallic phase, while -Pt2+-Pt3+-I-Pt2+-Pt3+-I- for the insulating one.