The thermochromic behavior of poly(di-n-octylsilane) {[Si(C8H17)(2)](n); PDOS] was studied by ultraviolet (UV) absorption, differential scanning calorimetry, and X-ray diffraction measurements. The structure of PDOS in the low-temperature phase strongly depended on not only the temperature but also the rate of cooling, that is, the thermal history. Temperature-dependent UV absorption spectra were highly dependent on thermal hysteresis. Cooled rapidly (10 K/min), PDOS showed two absorption peaks at 3.32 and 3.51 eV in low-temperature-ordered phases, whereas a single absorption peak at 3.32 eV became predominant with slow cooling (0.3 K/min). The appearance of the two peaks at low temperatures suggested that a mixture of different conformations was introduced by rapid cooling. A fiber diffraction pattern measured at 240 K after rapid cooling also showed evidence of the existence of novel conformation. A temperature-dependent powder X-ray diffraction pattern changed significantly between 270 and 280 K. Rapid cooling reduced the intensity of the X-ray diffraction peak in this temperature region. This intensity change was explained by the conformational mixture in the polymer.