The reactions of diethyl and di-n-propyl ethers have been studied in the presence of hydrogen on evaporated films of tungsten. In the temperature range from 200 to 260 $^\circ$C ethane and ethylene were formed from diethyl ether and small amounts of butenes were also produced. Each film had an initial high activity, especially for the formation of ethane, but the activity declined to a steady value during a transitional period. Subsequently, the decomposition of the ether occurred with zero order kinetics. A similar transitional period was observed during the decomposition of di-n-propyl ether but the change in the character of the reactions was more marked. Propane and propylene were formed initially, but very little further propane was produced after the initial period. If the surface of the tungsten was oxidized before exposure to ether, a high activity for the dehydration of the propyl ether was observed. Evidence from a number of experiments showed that irreversible changes were occurring to the catalyst during the transitional periods in which the metal surface was being converted to a different type of surface under the combined action of the ether and water vapour which was either added or formed by reaction. Most of the results could be interpreted on the assumption that two types of surface were formed-an oxidized surface of high activity for dehydration and an inactive surface covered by strongly adsorbed hydrocarbon residues. Subsidiary experiments were carried out with n-propanol on oxidized tungsten and evidence was found that the dehydration of the alcohol which was strongly adsorbed probably controlled the rate of reaction of the ether.