The synthesis of 1,1-diethoxyethane from ethanol and acetaldehyde was studied in a continuous flow reactor using Amberlyst 15 as catalyst. The resin underwent an important activity loss with both analytical quality reagents and industrial raw materials. The deactivation was related to a deterioration in acidity as a consequence of sulfonic group hydration mainly promoted by water from reaction. A higher ethanol conversion and a lower deactivation rate at long time on-stream were observed when n-hexane was added to the feed. (C) 2004 Society of Chemical Industry.