In this article we explore how multinational corporations (MNCs) adopt assurance practices to develop and sustain organizational accountability for sustainability. Using a panel of Fortune Global 250 firms over a period of 10 years, we document the diffusion patterns of third-party assurance of sustainability reports. We specifically investigate how evolving auditing practices, namely diversity of assurance standards and type of assurance providers, shape the quality of sustainability assurance statements. The results illustrate great variability in the adoption of assurance practices in (...) the formative stages of this novel market. Our descriptive analysis indicates the relevance of external institutional pressure as well as internal resources and capabilities as underlying factors driving the adoption of assurance. Our evidence also suggests that several MNCs project a decoupled or symbolic image of accountability through assurance, thereby undermining the credibility of these verification practices. The paper contributes to the emerging literature on international accountability standards and emphasizes the need to enhance theory-based, cross-disciplinary knowledge related to auditing and accountability processes for sustainability. (shrink)