Sulfur trioxide pyridine complex is the compound with the formula CHNSO.  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  +  CHNSO   →   [CHNH][ROSO]
It also is useful for sulfamations:
:RNH  +  CHNSO   →   CHN   +  RNSOH
The compound is used for sulfonylation reactions, especially in the sulfonylation of furans. It is also an activating electrophile in a Parikh-Doering oxidation.