Environmental organizations, characterized here as transnational advocacy networks, use various strategies to “green” international financial institutions (IFIs). This article goes beyond analyzing network strategies to examine how transnational advocacy networks reconstitute the identity of IFIs. This, it is argued, results from processes of socialization: social influence, persuasion and coercion by lobbying. A case study of the International Finance Corporation (IFC), as a member of the World Bank Group, is used to analyze how an IFI internalized sustainable development norms. The IFC finances private enterprise in developing countries by providing venture capital for private projects. Transnational advocacy networks socialized the IFC through influencing its projects, policies and institutions via direct and indirect interactions to the point where the organization now sees itself as a sustainable development financier. This article applies constructivist insights to the greening process in order to demonstrate how socialization can reshape an IFI's identity.