Lipid translocation in membranes is still far from being understood and well characterized for natural, cell membranes as well as for simpler bilayer model systems. Several; discrepancies with respect to its occurrence and its characteristic time scale are present in the literature. In the current work, the structural changes induced by lipid rearrangement in a distearoyl-/dimyristoyl-phosphocholine binary lipid system have been addressed by means of neutron reflectivity. It has been shown that a fast, spontaneous compositional reorganization with lipid transfer between the two leaflets of the bilayer takes place only when the lipid species are both in the fluid phase. This process has been identified as the so-called lipid flip-flop. Moreover, the influence of the preparation protocol on the structural properties of the system has been investigated.