We evaluate the effectiveness of impact investing from the perspective of the market failures approach (MFA) to business ethics. Under the MFA, businesses are ethically obligated to contribute to market efficiency by mitigating market failures. The MFA ethics literature emphasizes a negative externality interpretation of market failures, with ethical practice as self-regulation. We argue that the MFA also obligates businesses, and investors, to produce positive externalities, a form of private provision of public goods. We develop a graphical MFA ethical framework (...) addressed to impact investing. The framework is based on impact projects’ dual financial and social returns. Dual returns trade-offs originate in market failures and increase with positive externalities. In practice, a key determinant of market failure and the size of returns trade-offs is the availability of intermediate public goods. This varies systematically across sectors and country-markets. We identify the market circumstances under which impact investing is feasible. We show how provision of positive externalities mitigates market failures. We show that the effectiveness of impact investing depends on the interaction of the determinants of feasibility and of the size of project trade-offs. We show how government supporting intervention, through blended finance vehicles, can improve impact investing effectiveness. Finally, we show that evidence of actual patterns of impact investing across sectors and countries supports our analysis. (shrink)