Beta-amyloid peptide (A&beta) is the major protein constituent found in senile plaques in Alzheimer's disease (AD). It is believed that A&beta plays a role in neurodegeneration associated with AD and that its toxicity is related to its structure or aggregation state. It is important to understand the relationship between A&betaƒnstructure and function in order to better understand the toxicity behavior of this protein. MALDI-TOF Mass Spectroscopy coupled with Chemical Modification was used to identify residues on A&beta peptide that were exposed or buried upon changes in peptide structure associated with aggregation. The change in mass with chemical modification was used to indicate the solvent accessibility of specific peptide residues in different aggregation states. Results indicate that different relative abundance of fully accessible species and partially accessible species were observed in the different A&beta aggregation states: monomer and fibrils. Several residues were modified: Lysine, Tyrosine and Aspartic acid; limited proteolysis was performed to identify with accuracy which residue was accessible in the structure. Determine the specificity of residue modification and the rate of reaction for a specific site was the ultimate goal in order to understand how the peptide folds. Understanding A&beta peptide structure at the residue level is a first step in designing novel therapies for prevention of beta amyloid structural transitions associated with Alzheimer's disease.