X-ray acid neutron powder diffraction studies have been made on phase-pure LiNixMn2-xO4 (x = 0, 0.04, 0.18 and 0.33) substituted materials solely to establish how Ni substitutes into the spinel structure. Rietveld refinement showed all the materials to have the spinel phases (space group : Fd3m) with lithium occupying the tetrahedral (sa) sites, and Ni substituting for Mn at the octahedral (16d) sites. The cubic lattice parameter (a) decreases on Ni substitution : a = 8.251(4), 8.241(3), 8.219(3) and 8.196(3) Angstrom for x = 0.0; 0.04, 0.18 and 0.33, respectively.