We analyze the trajectory of independence in the formation of the European Central Bank (ECB), conceptualized as a boundary organization that, by delineating the European economy, contributes to a supranational state effect. Success in the effort, however, requires the ECB to constantly assert a separate and special status, despite its embeddedness in multiple fields. Focusing on the European Monetary Institute, the ECB's predecessor, we trace how historically obscure bank-based legal experts enabled the ECB's assertion of separateness by reworking independence into a newly multivalent category that could be wielded in authority struggles with national central banks and European institutions. The ECB's dependence on independence, we argue, renders it uniquely vulnerable to the repoliticization of central banking.