Solid sulfur trioxide has a trimeric ice-like structure: γ-SO3 [3]; however, a water content as low as 10−3 mol% alters this substantially, so that sulfur trioxide is most commonly encountered as the needle-like α-SO3 or β-SO3 phases, consisting of large polymeric chains HO(SO3)nH, which are also typical of high-concentration oleum. Oleum is most frequently produced as a SO3/H2O composition liquid at room temperature, corresponding to 0-30% or 65-75% free SO3, with the latter containing only ~5 wt% water. The liquid state frequently confers better mixing and increased reactivity, as well as making the reagent easier to manipulate (see Chapter 21).