Normal incidence X-ray standing wave (NIXSW) measurements are presented for sulphur terminated InP(100) surfaces, prepared by wet-etch in (NH4)(2)S solution followed by UHV anneal. Standing wave profiles for sulphur Is photoyield in three planes (220, 311 and 31(1)under bar) indicate that sulphur is close to the phosphorus site. Coherent positions and fractions are compared with the predictions of a number of models of the surface, including novel 2 x 2 structures. The experimental data can be reproduced by a general two-dimer model, but it is more likely that the data are reflecting significant disorder.