The structure of the Ni(100)c(2*2)Hg phase has been studied by soft X-ray methods. Delayed onset of photo-ionisation above the Hg MV and MIV edges prevented SEXAFS being used, but normal-incidence standing X-ray wavefield absorption at the (200) Bragg condition yields a Hg to Ni lattice plane spacing of 0.6+or-0.1 AA. This is consistent with bridge site adsorption and a Ni-Hg bond length characteristic of either the metallic radii or the Ni-Hg compound. The alternative hollow site can only be reconciled with the data if the surface Ni-Hg bond length is significantly longer, probably accompanied by some top Ni layer relaxation.