Sulfur trioxide (SO3) is a key precursor to sulfate and present in oleum, a solution of sulfur trioxide 25-65 % in sulfuric acid (H2SO4). Moreover, sulfur trioxide is a strong Lewis acid, engages in reactions with Lewis bases such as trimethylamine (Me3N), triethylamine (Et3N), dioxane, and pyridine (Py). These reactions result in the formation of sulfur trioxide adducts, which are be used in the sulfation process of various organic substrates, leading to the formation of organosulfate esters (Figure ). Figure : Sulfur trioxide complexes with trimethylamine, triethylamine, dioxane, and pyridine.