Although the interaction between the β-amyloid peptide and copper (II) appears to play an important role in Alzheimer’s disease, the affinity constant is still controversial and values are ranging from 107 to 1011 M−1. With the aim of clarifying this point, a complementary method, based on the capillary electrophoresis–ICP–MS hyphenation, was developed and competitive binding experiments were conducted in the presence of nitrilotriacetic acid. The effect of the capillary surface (neutral or positively charged) and nature of the buffer (Tris or Hepes) have been studied. Tris buffer was found to be inappropriate for such determination as it enhances the dissociation of copper (II) complexes, already occurring in the presence of an electric field in capillary electrophoresis. Using Hepes, a value of 1010 M−1 was found for the affinity of the small β-amyloid peptide 1–16 for copper (II), which is in agreement with the values obtained for other proteins involved in neurodegenerative diseases. These constants were also determined in conditions closer to those of biological media (higher ionic strength, presence of carbonates).