The assembly of the amyloid β-protein (Aβ) into neurotoxic oligomers and fibrils is a seminal pathogenic process in Alzheimer's disease (AD). Understanding the mechanisms of Aβ assembly could prove useful in the identification of therapeutic targets. Owing to the metastable nature of Aβ oligomers, it is difficult to obtain interpretable data through application of classical methods, such as electrophoresis, chromotography, fluorescence, and light scattering. Here, we apply the method Photo-Induced Crosslinking of Unmodified Proteins (PICUP) to the study of Aβ oligomerization. This method directly produces covalent bonds among unmodified polypeptide chains through in situ generation of peptide free radicals. PICUP provides a snapshot of the native oligomerization state of proteins and can be used for assembly state analysis of a wide variety of peptides and proteins.