The remarkable growth of Sardex as a local currency throughout the island of Sardinia over the past 4 years motivated an in-depth look at its starting assumptions, its design and operational principles, and its socio- economic context. The paper looks at Sardex as a social innovation start-up, as a medium of exchange and unit of account, as an online and offline mutual credit system, and as a closed economic community or ‘circuit’. The analysis relies on interviews of circuit members conducted in the Summer of 2014 and benefits from the reflexive point of view of one of its founders. The main findings are that trust was and continues to be fundamentally important for the creation and operation of the local Sardex currency and of the circuit, and that Sardex mediates both economic and social value(s). These properties make it an ideal space for experimentation in socio-economic innovation that can be characterized as a ‘laboratory of institutional learning.