This paper presents a longitudinal case study examining why and how commercial banks sought to integrate sustainability issues into their project finance operations between 2003 and 2008. We study the evolution of a set of influential environmental and social risk management guidelines for project finance - the Equator Principles (EP) - and the simultaneous structuration of a field around these guidelines focused on the issue of socially accountable project finance. The case is theoretically framed using Hoffman’s (1999) concept of an issue-based field and associated conceptualisations of the role of internal and external (social) movements in the structuration of these fields. The structuration of the issue-based field studied is shown to encompass a dynamic, contested process involving extensive interactions between a non-governmental organization (NGO) movement and a commercial bank movement. We unveil how the conflicting, collective rationales and actions of both movements fuelled the structuration process and facilitated an evolution in the social accountability of commercial banks. While prior work sees little potential for civil society actors to engage with and move corporate social responsibility and reporting in a more challenging direction, we reveal how the NGO movement evoked a progression in social responsibility and reporting in a sector that had previously shown little inclination to address its wider social accountability. Drawing on our case analysis, we theorise how issue-based fields cohere and crystallise, particularly how they build an institutional infrastructure based upon the infrastructure of the mature field which they straddle and which the relevant issue impacts upon.