A situation in which molecules of @[email protected] are free to move about the surface. @[email protected] of the @[email protected] will increase with temperature and mobile adsorption may be either localized or non-localized. In localized mobile adsorption the @[email protected] spends most of the time on the adsorption sites but can migrate or be desorbed and readsorbed elsewhere. In non-localized adsorption the @[email protected] is so great that a small fraction of the adsorbed species is on the @[email protected] sites and a large fraction at other positions on the surface.