This article conceptualizes financial transactions as parts of financial infrastructures. Not only do transactions perform services for the economy, mainly in the area of calculation and pricing, but there is also merit in a conceptually infrastructural view on transactions which uncovers their ambivalence for the stability of the financial system. This is based on a conceptualization of infrastructures that distances itself from inherited modernist notions of the completeness, full operability, and functional integration of infrastructures, and instead highlights the constitutive incompleteness, error-proneness, and looming disintegration of infrastructures owing not to external threats but to their very modes of operation. The article analyzes two post-crisis reports that try to sort out this infrastructural ambivalence of transactions, and, in that attempt, mobilize different imaginaries. In the Brady Report following Black Monday of 1987, the imaginary of the efficient competitive market was cited to stabilize the boundary between functional and dysfunctional transactions. In the FCIC Report reflecting on the subprime mortgage crisis, a quasi-sociological diagnosis of mushrooming, and morally problematic transactional relationships were invoked to separate functional from dysfunctional intermediation by financial transactions.