The Social Impact Bond (SIB) is a new funding mechanism for welfare programs. It is supposed to create savings for the public sector from which private returns can be deducted. Presented as a purely technical solution, SIBs discard their political morality. The Welfare Convention Approach (WCA) is designed for studying SIBs as disputed and versatile welfare apparatuses. It is claimed that elements from diverse historical welfare conventions (the philanthropic, communitarian, civic, market, full employment, entrepreneurial, financial, and behavioral) reveal the diverse institutional conflicts and compromises of SIBs at the time they are implemented. In so doing, the WCA informs comparative research on SIBs.