Abstract There have been few systematic investigations of the physico-chemical properties of the more important plant proteins; such investigations as have been carried out in the past have concerned themselves almost entirely with the better known animal proteins. It was, therefore, thought desirable to make a study of the physico-chemical properties of the protein gliadin, which, together with glutenin, forms the chief constituent of the gluten of flour. Of all the methods employed for determining the mobility of colloid ions in an electric field, the micro-cataphoretic method is the most exact. In studying the micro-cataphoresis of protein sols in which the particles are not visible in the ultramicroscope, it has been usual to make measurements using inert particles coated with protein. Although it was found possible to obtain gliadin sols containing visible submicrons, it was decided to make observations also on quartz particles covered with gliadin.