The compounds (TTM-TTP)(I-3)(5/3) and (TSM-TTP)(I-3)(5/3) which have polyiodide chains along the donor stacking direction and show characteristic ''copper'' luster, exhibit metallic behavior above T(MI)approximate to 20 K. High pressure resistivity, the Raman spectra, and optical reflectance of these salts have been measured to investigate the origin of the low-temperature insulating state and the iodine species. For (TSM-TTP)(I-3)(5/3) two kinds of conducting behavior have been observed; below 20 K some low-conducting samples have shown an increase of the resistivity of more than 10(3) times, but other high-conducting samples have shown an increase of less than 10 times. The increase of resistivity is almost suppressed under a pressure of 11.5 kbar in the high-conducting phase of (TSM-TTP)(I-3)(5/3). The Raman spectra provide clear evidence that the polyiodide chain is composed of I-3(-) in these compounds. The Raman spectra and the x-ray photographs indicate that the increase of resistivity originates in the disorder. The chain axis optical reflectance spectra show plasma edges appearing in the infrared region and three peaks from the infrared to the visible range; the latter originate from the intramolecular transition and polyiodide ions. Temperature dependence of optical conductivity is metallic even below T-MI. From these results, the origin of the low-temperature insulating state is attributed to disorder driven localization. [S0163-1829(99)06331-6].