Amyloid-ß (Aß) is a peptide of 39-43 amino acids that is implicated in the pathology of Alzheimer's disease: its misfolding results in the formation of a diverse array of toxic aggregates. Recent results show that soluble oligomers – not larger, fibrillar species – are the most cytotoxic of these. Therefore, detailed understanding of the earliest peptide-association steps is crucial to Alzheimer's prevention and treatment. We have utilized single-molecule spectroscopy (SMS) to monitor oligomer formation in Aß(1-40), which is labeled at the N-terminus with FAM fluorescent dye and tethered to functionalized cover slips via biotin-streptavidin binding. Since each dye exhibits a characteristic fluorescence intensity, the number of Aß monomers in one peptide species was easily deduced from the number of discrete intensity levels it displayed prior to photo-bleaching. By investigating tens of tethered peptides, one at a time, we have directly determined the distributions of monomers and small oligomers present in heterogeneous solutions. Fresh samples at pH 7.4 consist primarily of monomers and dimers, with a small percentage of trimers. After aging the stock solution for 5 days, the proportions of dimers and trimers are slightly increased. At pH 5.7, peptide association is rapid: fresh samples are dominated by trimers and tetramers, with fewer dimers and larger species. These results agree well with expectations based on published bulk-solution studies and illustrate the power of SMS for characterizing small Aß oligomers and testing inhibitor compounds against the earliest aggregates.