Oligomeric alpha-synuclein is considered to be the potential toxic species responsible for the onset and progression of Parkinson's disease, possibly through the disruption of lipid membranes. Although there is evidence that oligomers contain considerable amounts of secondary structure, more detailed information on the structural characteristics and how these mediate oligomer-lipid binding are critically lacking. We have used tryptophan fluorescence spectroscopy to gain insight into the structural features of oligomeric alpha-synuclein and the structural basis for oligomer-lipid interactions. Several single tryptophan mutants of alpha-synuclein were used to gain site-specific information about the microenvironment of monomeric, oligomeric and lipid bound oligomeric alpha-synuclein. Acrylamide quenching and spectral analyses indicate that the tryptophan residues are considerably more solvent protected in the oligomeric form compared to the monomeric protein. In the oligomers, the negatively charged C-terminus was the most solvent exposed part of the protein. Upon lipid binding a blue shift in fluorescence is observed for alpha-synuclein mutants where the tryptophan is located within the N-terminal region. These results suggest that as in the case of monomeric alpha-synuclein, the N-terminus is critical in determining oligomer-lipid binding. We have further systematically studied the influence of the physical membrane properties and solution conditions on lipid bilayer disruption by oligomeric alpha-synuclein using a dye release assay, and have quantitatively studied oligomer lipid binding by confocal fluorescence microscopy and fluorescence correlation spectroscopy (FCS). The results indicate that the oligomeric species specifically bind to negatively charged lipids in the liquid disordered phase.