The results of investigations on the phenomenon of migration in films of alkalis on relatively inert metals fall naturally into two classes. Those authors who have worked on the thermionic properties of the composite surface, and who, as a result of the high temperatures necessarily employed, have usually been concerned with dilute films, have recorded that a relatively large activation energy is required for migration. Those, on the other hand, who have used a photo-electric method of observing change in the surface and have on that account usually dealt with relatively close packed (and so more photo-sensitive) films, have recorded measurable mobility at quite low temperatures. Thus Lukirsky and Ryanoff believe as a result of their experiments on the photo-sensitization of potassium by hydrogen that the surface film is mobile at a temperature as low as 210°K. Kollnert found that the oxidation of caesium at 93°K enhances the photo-sensitivity owing, he claims, to the formation, by a process of surface migration, of a monolayer of caesium over the caesium oxide formed as a result of primary oxidation. More recently the writer found that films of sodium a little thicker than a monolayer had quite a high mobility on tungsten at 300°K. It has been recorded in a few instances that the mobility of films of foreign material rises with surface concentration. Thus Taylor and Langmuir examining dilute films of caesium on tungsten found that at 967° K a fourfold increase in the concentration resulted in an elevenfold increase in the surface diffusion coefficient D. Becker and Brattain’s work on the surface diffusion of thorium on tungsten also indicated an increase of D with the surface concentration. This increase was due, they suggested, to partial evaporation followed by recondensation. Langmuir pointed out that this migration was not assisted by a strong negative field around the filament. We should expect such a field to assist the evaporation of positive ions and consequently expedite surface diffusion. We are thus led to postulate the existence of a surface spreading force F originating in the mutual repulsion of the adsorbed positive ions, which, rising with increasing concentration, serves to assist any surface movement which would result in the equalization of surface concentration. The existence of this spreading force has been emphasized by Langmuir, who points out, in particular, that F and therefore D should rise extremely rapidly as θ, the fraction of the surface covered, tends to unity. It may, however, also be observed that the activation energy, E, associated with the diffusion process should fall with rising θ on account of the lowering of the effective potential barrier opposed to surface diffusion by means of the potential of the spreading force F. A knowledge the functional connexion between E and θ would enable F to be calculated and the nature of the repulsive forces acting between adsorbed positive ions elucidated.