A. Allescher-Last, A. Czybulka, H.- Crystal truncation rod / Fourfold hollow site j On-top site \ Surface structureAbstract. The analysis of the intensity along the diffraction rods normal to the crystal surface (crystal truncation rods) has been used to analyse the adsorption sites of potassium atoms adsorbed at T= 330 Κ on atomically clean Cu(100) under ultra high vacuum conditions. At potassium coverages below about Θ = 0.18 ML (1 ML = 1.53 χ IO 15 atoms cm" 2 ), where no ordered adsorbate superstructure forms, the potassium atoms are adsorbed in the fourfold hollow sites of the Cu(100) substrate at d L = 2.1(4) Â above the first Cu layer corresponding to an effective potassium radius of r cff = I. 5(3) Â which is close to the ionic radius of 1.33 Â. Within about 1 -2% the substrate interlayer spacings, d l2 and d 2 z are not affected.At high coverage (Θ % 0.30 ML) where an ordered incommensurate overlayer forms, the adsorption site is partially on-top. These results are in correspondence with the model of alkalimetal adsorption on metals supposing extensive charge transfer from potassium to the substrate at low coverage.