Sulfur trioxide pyridine complex is the compound with the formula C5H5NSO3. It is a colourless solid that dissolves in polar organic solvents. It is the adduct formed from the Lewis base pyridine and the Lewis acid sulfur trioxide. The compound is mainly used as a source of sulfur trioxide, for example in the synthesis of sulfate esters from alcohols: ROH + C5H5NSO3 → [C5H5NH]+[ROSO3]− It is also an activating electrophile in a Parikh-Doering oxidation.