Abstract: A method for reacting sulfur trioxide and sulfateable or sulfonateable liquid organic compounds such as alcohols, and unsaturated or aromatic compounds is disclosed which comprises contacting said organic liquid with a gaseous mixture of from 2 to 20% by volume of sulfur trioxide and the balance an inert gas diluent, in a vertical cylindrical reaction zone that is at least about 25 mm. in diameter and comprises a straight cylinder having a gas-liquid contact length of at least 1.5 meters and having an annular inlet means for the liquid at the lower end of the reaction zone; the contact being effected by means of an upward stream of the gaseous mixture having sufficient velocity to form the liquid into a rising annular film throughout the length of the zone.