This study examined whether (1) assurance, (2) the level of assurance (reasonable vs. limited), and (3) the type of assurance provider (accountant vs. specialist consultant) affect users' perceptions of reliability of sustainability reports. Based on an experimental questionnaire, we find that the provision of assurance improves report users' perceived reliability of the environmental and social information. There were no significant main effects between the two experimental factors; level of assurance and type of assurance provider, and report users' perceptions of the reliability of sustainability reports. However, a significant interaction was found between the two experimental factors and report users' perceptions of reliability of such reports. More specifically, report users placed more confidence in the sustainability reports where the level of assurance provided is reasonable (high), and when such assurance is provided by a top tier accountancy firm, rather than when the assurance is provided by a specialist consultant. No such difference was found when the level of assurance provided was limited (low) for either type of assurance provider group. The results of this study thus highlight the relevance of assurance for sustainability reporting.