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) (Aβ10-35) and a block copolymer β-Amyloid(10-35)-PEG3000 (Aβ10-35-PEG) in D2O at pHs from 3.0 to 7.0. These studies indicate that Aβ10-35 aggregates into rod-like particles (fibril) and their radii are strongly dependent on the pH of the solution. The fibril-fibril association in Aβ10-35 solutions is less at pH < 5.6, but becomes larger at higher pH. Aβ10-35-PEG also assembles into rod-like particles whose radius is larger by about 30 Å than that for Aβ10-35 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 Aβ10-35 and Aβ10-35-PEG fibrils at pH 5.6. The mass per unit length of the rods from SANS provides key information on the packing of Aβ10-35 peptides in the fibril.