The relative effective areas of krypton and xenon atoms adsorbed on iron, cobalt, nickel, titanium, zirconium, molybdenum, tungsten, tantalum and platinum have been examined by determination of the 77 degrees K isotherm for each gas adsorbed successively on the clean surface of the same evaporated film. Point A, point B and the B.E.T. equation have been used to assess the monolayer value, with the general result that the number of atoms required to complete the monolayer is the same for xenon as for krypton, despite the larger atomic size of xenon. This result is interpreted to mean that both kinds of atoms occupy the same sites and that their effective area is determined by the site area. The more usual supposition that adatoms are close-packed, each exerting a characteristic area independent of the nature of the surface, is rejected. Various configurations of adatoms are examined with the object of finding the densest occupation of available sites. For this purpose, interatomic energies have been calculated using the Lennard-Jones (6:9) and (6:12) potentials and the Buckingham exp:6 potential. In several instances, clear decisions can be reached, and the general conclusion is that equal numbers of krypton and xenon atoms are required to form the monolayer, in keeping with experiments.