© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group This study examines how financial audit-styled concepts such as materiality are transferred to non-financial audit arenas. Drawing on a case study of assurors working within a Big 4 professional services firm, we uncover a number of interrelated features of the materiality determination and assessment process within sustainability assurance (assurance on sustainability reports). We illustrate how assuror flexibility, underpinned by assuror intuition, is central to uncovering assurance technologies deemed capable of addressing the materiality of ambiguous sustainability data. Assurors with no financial audit background retrospectively rationalise their intuition using the assumed authority of structured financial audit methodologies. This facilitates the tentative translation of financial audit knowledge to the sustainability assurance domain. Collaborative, holistic decision-making processes inform the assurors’ continual construction of materiality and are characterised by alliances of (accountant and non-accountant) ‘expert’ assurors merging formal and tacit knowledge. These alliances seek social cohesion within sustainability assurance teams in order to establish a social consensus among assurors around the materiality determination and assessment process. Our analysis develops and extends Power’s theorisation of how new areas are made auditable and advances our understanding of the more practical aspects of non-financial assurance services offered by Big 4 professional services firms.