This paper investigates the detail and dynamics of how contractual and relational governance mechanisms are deployed in managing complex, long-term public–private supply arrangements. Using empirical data from two UK Private Finance Initiative (PFI) cases, the paper analyses the interplay of governance mechanisms along a timeline of project phases. Conceptual and practical findings confirm that relational and contractual mechanisms are indeed complementary forms of exchange governance but also that the relational intentions of all parties frame whether the contract is interpreted as a written sign of distrust or commitment. Equally, relational and contractual governance follow different development paths: both follow cumulative trajectories but (a) inter-personal relational mechanisms are more incremental and fragile, whereas (b) contractual mechanisms move with fewer degrees of freedom and ‘anchor’ the exchange throughout the life cycle. Finally, the different development characteristics of relational and contractual mechanisms mean that their dynamic interplay does not follow consistent patterns. The paper concludes with suggestions for more longitudinal studies.