AbstractDepolarization ratios ρ of the Raman bands due to CH3stretching at 2907 cm−1and the SiO skeletal mode at 491 cm−1have been measured in polydimethylsiloxane gum as a function of temperature from 100°C to −45°C. Below 0°C the changes inphave been interpreted in terms of the formation of helical regions in the gum. The enthalpy of helix formation ΔHhas been determined as 3200 ± 600 cal/mole. An upper limit on the entropy change, ΔS, of 16 ± 3 e.u./mole and minimum values of helix content at different temperatures have been found. The Raman spectrum of crystalline polydimethylsiloxa