Insulin-degrading enzyme (IDE) hydrolyzes monomeric polypeptides, including amyloid-β (Aβ) and HIV-1 p6. It also acts as a nonproteolytic chaperone to prevent Aβ polymerization. Here we compare interactions of Aβ and non-amyloidogenic p6 with IDE. Although both exhibited similar proteolysis rates, the binding kinetics to an inactive IDE characterized using relaxation-based NMR were remarkably different. IDE and Aβ formed a sparsely populated complex with a lifetime of milliseconds in which a short hydrophobic cleavage segment of Aβ was anchored to IDE. Strikingly, a second and more stable complex was significantly populated with a subsecond lifetime owing to multiple intermolecular contacts between Aβ and IDE. By selectively sequestering Aβ in this nonproductive complex, IDE likely increases the critical concentration required for fibrillization. In contrast, IDE and p6 formed a transient, submillisecond complex involving a single anchoring p6 motif. Modulation of intermolecular interactions, thus, allows IDE to differentiate between non-amyloidogenic and amyloidogenic substrates.