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A monolayer of H atoms adsorbed on Ni(ll0) below 180 K forms a (2 • 1) structure. The unit cell exhibits a glide symmetry plane and contains two adsorbed atoms. Based on a quantitative comparison between experimental and calculated LEED I/V spectra using standard R-factors the following structure was derived: On the clean Ni(ll0) surface the separation between the first two atomic layers, da2, is contracted by 8.5% + 1.5% with respect to the bulk value; those between the second and third and the third and fourth layer, d23 and d34, are expanded by 3.5% + 1.5% and 1% + 1.5%, respectively -in agreement with recent other resutls. In the presence of the H adlayer the contraction of da2 is reduced to 4.5% :i:1.5%, while the expansion of d23 is not affected within the limits of accuracy. The third interlayer spacing d34 returns to its bulk value. The H atoms occupy threefold-coordinated sites formed by two Ni atoms from the first layer and one Ni atom from the second layer which confirms previous more qualitative conclusions based on He diffraction and vibrational spectroscopy. The bond lengths between H and its neighbouring Ni atoms were determined to be equal, namely 1.72+0.1 ,~.The structure of adsorbate layers is of particular interest for the understanding of the energetics of the adsorption system; for the substrate-adsorbate interaction determining the local adsorption geometry as well as for the adsorbate-adsorbate interactions which govern the lateral distribution of the adparticles. Under these aspects the system H/Ni(ll0)[1] appears extremely promising because of its large number of ordered structures. At T< 180 K the adsorbed H atoms form a series of ordered lattice gas structures, which are completed with a (2 • 1) phase at 0 = 1.0 [2,3]. Above this coverage Ha0 induces a reconstruction of the surface into domains of a (1 • 2) structure [4] which covers the entire surface at a saturation coverage of * Present address: Fritz-Haber

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