Social Impact Bonds are considered a highly marketised form of public service delivery and are understood to 'work' through the introduction of new capital into payment-by-results contracts. This paper, for the first time, connects findings from UK SIBs to evaluation of conventional payment-by-results contracts and the theoretical literature on governance and accountabilities. Markets and capital emerge as a potential red herring with hybridity of governance and the 'social' positioned as important dimensions facilitating qualitatively different services. This raises questions as to whether it is possible to extend SIBs, or whether increased scale and mainstream investors will dilute the 'SIB effect'.