Alzheimer's disease is characterized by the presence of insoluble, fibrous deposits composed principally of amyloid ␤ (A␤) peptide. A number of studies have provided information on the fibril structure and on the factors affecting fiber formation, but the details of the fibril structure are not known. We used fluorescence quenching to investigate the solvent accessibility and surface charge of the soluble A␤(1-40) dimer and amyloid fibrils. Analogs of A␤(1-40) containing a single tryptophan were synthesized by substituting residues at positions 4, 10, 34, and 40 with tryptophan. Quenching measurements in the dimeric state indicate that the amino-terminal analogs (A␤F4W and A␤Y10W) are accessible to polar quenchers, and the more carboxyl-terminal analog A␤V34W is less accessible. A␤V40W, on the other hand, exhibits a low degree of quenching, indicating that this residue is highly shielded from the solvent in the dimeric state. Correcting for the effect of reduced translational and rotational diffusion, fibril formation was associated with a selective increase in solvent exposure of residues 34 and 40, suggesting that a conformation change may take place in the carboxyl-terminal region coincident with the dimer to fibril transition.