Abstract: A method of gaseous acid catalysis which finds particular applications in the manufacture of furfural includes the steps of introducing a solid reactant containing one or more hydroxyl groups into a reactor (1); introducing superheated steam from a superheater (2) until the temperature within the reactor (1) is higher than that of the dew points of both water and the catalyst and the reactant is dry. The catalyst is then introduced into the superheated steam by vaporiser (3). Product gas formed is liquified in condenser (4) and the condensate collected in a buffer tank (5). Separation plant isolates the product gas and recovers the acid catalyst, preferably as its azeotrope with water, and recycles it into the vaporiser.