Labour-controlled investment is often touted as an alternative, pro-worker form of finance. Since 1983, the province of Québec in Canada has experimented with workers participation in the form of workers funds controlled by the two major trade union federations. Drawing on research from secondary sources, archival material and semi-structured interviews, this paper offers a comprehensive portrait of one of Québec main workers' funds, the FTQ Solidarity Fund. To date very little has been said about the impact of workers funds on firm governance, employment quality and labour relations. We argue that any attempt to use investment to shape firm behaviour in the interests of workers and local unions is a limited and contradictory project. The argument is sustained through a discussion of the historical formation and institutional practices of the Solidarity Fund. The presence of large union-controlled investment funds offers local firms an alternative source of capital investment, protecting small and medium-sized firms from more aggressive financial actors. However, our main results show persistent tensions and contradictions between the Fund's social goals and financial logics, as well as unintended effects on workplace union practices and power.