The structure of the Cu(111)-(3x3)-SiHx surface formed by reaction of silane (SiH4) with Cu(111) at 140 K, has been determined using normal incidence X-ray standing wave (NIXSW) analysis. The (3 x 3) phase, thought to comprise a mixture of SiH3 (2/9 ML) and SiH or Si species (1/9 ML), has all the surface silicon atoms located in hcp 3-fold hollow sites at a distance of 1.98+/-0.04 Angstrom from the surface copper scatterer planes. (C) 2002 Published by Elsevier Science B.V.