Small angle neutron and x-ray scattering (SANS/SAXS) studies were conducted on the structure of the aggregates formed from both the truncated model peptide β-Amyloid(10-35) (Aβ_10-35) and a block copolymer β-Amyloid(10-35)-PEG3000 (Aβ_10-35-PEG) in D₂O at pHs from 3.0 to 7.0. These studies indicate that Aβ_10-35 aggregates into rod-like particles (fibril) and their radii are strongly dependent on the pH of the solution. The fibril-fibril association in Aβ_10-35 solutions is less at pH < 5.6, but becomes larger at higher pH. Aβ_10-35-PEG also assembles into rod-like particles whose radius is larger by about 30 Å than that for Aβ_10-35 fibril at pH 4.2, while it is about 23 Å larger at higher pH. Contrast matching SAXS/SANS experiments that eliminate the coherent scattering from PEG reveal that PEG moiety is located at the periphery of the fibril. Also the mass per unit length of the peptide portion is similar for both Aβ_10-35 and Aβ_10-35 fibrils at pH 5.6. The mass per unit length of the rods from SANS provides key information on the packing of Aβ_10-35 peptides in the fibril.