sulfur trioxide in neat (no solvent) byproducts: pyridinium fluorosulphate; stirring complex salt in flask; evacuation; introduction of SO3 vapour in stages; keeping reactants in contact for 15-20 mins after addn. of last aliquot of SO3; cooling with liquid N2, then with alcohol bath at -50°C; apparatus described;; warming to room temp.; pumping free from excess SO3; filling vessel with dry N2; detn. by IR, 19F-NMR; elem. anal.;;