Poly(n-butyl-n-hexylsilylene) (PBHS) has been studied by UV and Raman spectroscopy as a function of temperature from 300 to 27 K. At room temperature, PBHS adopts a hexagonal columnar mesophase. Upon cooling to 250 K or below, a complex first-order phase transformation takes place, leading to three other forms : a 7/3 helical form, an all-trans form, and an intermediate form with unknown conformation. The amount of each form depends on the thermal history of the sample. The final transformation, from 7/3 helical to the all-trans conformation, does not proceed to completion because equilibration ceases at the glass transition temperature, near 210 K.