Biological membranes are the initial targets of pollutants from the surrounding environment. Understanding the mechanisms controlling their interactions at the molecular level is of primary importance to interpret the results of bio-accumulation and toxic effects. Mercury is a major environmental contaminant, which has been shown to cause increased morbidity and mortality in humans. In order to understand the molecular mechanism of mercury toxicity with cell membranes HgCl2 and CH3HgCl were made to interact with multilayers built up of dimyristoylphosphatidylcholine (DMPC) and dimyristoylhosphatidylethanolamine (DMPE), which represent classes of phospholipids located in the outer and inner monolayers of the human erythrocyte membrane, respectively. Results presented in this paper indicated that both mercuric compounds interacted with and perturbed the molecular structures of DMPC and DMPE. These studies were performed by X-ray diffraction methods