β-2-Microglobulin (β2m) self-associates into fibrillar amyloid deposits in the musculoskeletal system of patients undergoing hemodialysis treatment. Previous studies have shown that stoichiometric amounts of Cu(II) at near physiological conditions can cause β2m to organize into native-like dimers prior to forming amyloid fibrils. Here, we report the results from selective covalent labeling reactions combined with mass spectrometry that provide insight into the amino acid residues that mediate dimer formation in the wild-type protein. Using three complementary covalent labeling reagents, we find that the dimer interface is formed by the antiparallel stacking of ABED β-sheets from two β2m monomers. In addition, our data clearly indicate that a dimer interface involving the interactions of D−D strands from separate protein units as seen in the recent crystal structures of two mutant β2m oligomers is unlikely.