This paper explores the factors associated with voluntary decisions to assure social, environmental and sustainability reports. Since the market for assurance services in this area is in its formative stages, there is a limited understanding of the demand for this emergent non-financial auditing practice, which is evolving rapidly across different countries. Drawing from extant literature in international auditing and environmental accounting, we focus on a set of country-level institutional factors to explain the adoption of sustainability assurance statements among an international panel of 212 Fortune Global 250 companies for the years 1999, 2002 and 2005. Consistent with our expectations, our results provide evidence that companies operating in countries that are more stakeholder oriented and have a weaker governance enforcement regime are more likely to adopt a sustainability assurance statement. Further, the demand for assurance is higher in countries where sustainable corporate practices are better enabled by market and institutional mechanisms. Our exploratory findings also indicate that the likelihood of choosing a large accounting firm as assurance provider increases for companies domiciled in countries that are shareholder oriented and have a lower level of litigation. We conclude the paper by suggesting three directions of research in the area of sustainability assurance that have relevant academic and practical implications. Copyright © 2008 John Wiley & Sons, Ltd and ERP Environment.