Lead Magnesium Niobate (PMN) displays a superstructure characterized by the presence of broad half-order diffraction peaks. The structure is known to involve chemical ordering between alternate B sites within small domains. It has been proposed that this might be due to small domains of Mg:Nb chemical ordering with full 1:1 stoichiometry. Such order is inconsistent with the bulk composition ofPbMg1/3Nb2/3O3and would lead, moreover, to a net charge of−12eper unit cell. In this work we report our resonant X-ray diffraction experiments designed to distinguish 1:1 from bulk-stoichiometric ordering. ©2000 American Institute of Physics.