The first step of HIV infection involves the fusion of the viral and target cell membranes, a process mediated by the viral envelope glycoproteins, gp120 and gp41. The binding of gp120 to the cell surface receptors CD4 triggers a cascade of conformational changes that disrupt the gp41-gp120 interactions and allows the insertion of the N-terminal fusion peptide of gp41 into the host cell membrane. The gp41 trimer then rapidly folds into a 6-helix bundle that pulls together the fusion peptide, inserted in the host cell membrane and the transmembrane domain, located in the viral membrane. Very little structural information is known about the pre-fusion state of gp41 notwithstanding its critical importance for the design of fusion inhibitors. To investigate the dynamics and structural properties of such metastable states, we designed a set of protein constructs mimicking the extracellular ectodomain of gp41. We found that the secondary structure of gp41 ectodomain is pronouncedly impacted by the presence of phospholipid vesicles or detergents containing phosphatidyl choline head groups. Our NMR and multi-angle light scattering data attribute these changes to the transition between a trimer in the absence of detergent to a monomer in the presence of a membrane-mimicking environment. The structure of gp41 in a monomeric state was determined by state-of-the-art NMR techniques, including measurements of NOE distance restraints and residual dipolar couplings in weakly aligned solutions. 15N relaxation data provided a detailed view at the backbone dynamics of both the trimeric and monomeric conformations. This stable monomeric state may represent a crucial structural intermediate facilitating both the gp41 conformational change during fusion and the local apposition of the viral and cellular membranes.