Poly(di-n-pentylsilane) has been previously reported to adopt a 7/3 helical conformation at room temperature. We have studied the solid state structure of a low molecular weight (M(w) = 46 000) sample and its dependence on temperature using wide angle X-ray diffraction, differential scanning calorimetry, and Raman spectroscopy. The results suggest that domains of two distinct structures are present, one having an all-trans conformation and the other a 7/3 helical conformation. The all-trans conformation was observed (even at room temperature) in samples cooled previously to -15 degrees C. Characteristic resonance enhancement of certain Raman vibrations is observed for the planar zigzag polymorph. The all-trans material converts to the 7/3 helical structure at 35 degrees C and subsequently undergoes an order/disorder transformation at about 56 degrees C.