Despite having been around for a decade now, Social Impact Bonds (SIBs) payment by result contracts funding social programmesare still a niche instrument. Constituting but a fraction of the overall impact investment sector, they were expected to grow much faster and augur a new model of pursuing social policy objectives. Whilst this has not yet occurred, they nevertheless continue to benefit from a great degree of political support and academic interest. But outside of the practitionerfocused literature, the scholarship investigating SIBs has largely identified financialisation and the erosion of social solidarity as the main dynamics underpinning this development. This article argues that it is important to also attend to SIBs as expressions of transformations occurring within the design and pursuit of social policy objectives. By looking at SIBs as a form of governance of social risks, the article argues that SIBs nurture their own forms of social solidarity. Based on three distinguishing tenets of SIBs, three types of solidarities are emphasised: inter-temporal, cross-sectoral and risk-insurance solidarities. Whilst these can spur social inclusion, innovation and collaboration, the article discusses how they can also be spurious and can come undone.