Thionyl chloride has a long shelf life, however "aged" samples develop a yellow hue, possibly due to the formation of disulfur dichloride. It slowly decomposes to SCl, SO and Cl at just above the boiling point. Thionyl chloride is susceptible to photolysis, which primarily proceeds via a radical mechanism. Samples showing signs of ageing can be purified by distillation under reduced pressure, to give a colourless liquid.