Abstract In recent years a large amount of evidence has accumulated showing that the range of molecular attraction is very small; hence the specific catalytic action of each substance must decrease very fast as the distance of the reacting molecules from the surface increases. The catalytic action must therefore be confined to the surface layer of adsorbed molecules alone. When alcoholic substances react at copper surfaces below 280° C., only the — CH2OH group is changed. The only molecules which react, therefore, are those which have the — CH2OH group in contact with the copper. The behaviour of cetyl alcohol on water, and on glass and steel, shows that the hydroxyl group is the active agent in orientating the film structure.It is known also that alcohol will displace hydrocarbons from copper foil. The polar hydroxyl group is attracted to the copper surface more strongly than is the hydrocarbon chain. The evidence, therefore, shows that not only are the molecules that react adsorbed with the hydroxyl group in contact with the surface, but that the whole layer in contact with the catalyst is orientated in this manner. It may be that the molecules in the successive layers are also arranged similarly, but the evidence is scanty. In the case of the fatty acids, X-ray analysis gives confirmation of this arrangement