The atomic structure of the Ni(110)4 x 1-S reconstruction has been det
ermined on the basis of surface X-ray diffraction measurements. An ana
lysis of the in-plane diffraction data shows that the model consists o
f Ni rows along the [001] direction, two for every 4 x 1 unit cell, co
rresponding to 0.5 ML Ni coverage. The S is chemisorbed in pseudo two-
fold hollow sites both on the Ni rows and in the troughs between the r
ows with a S coverage of 0.75 ML. Furthermore, rod-scans along fractio
nal-order reflections reveal sub-surface relaxations. The results are
in good agreement with recent STM and previous AES and radioactive tra
cer studies.