This study examines whether (1) assurance, (2) the level of assurance (reasonable vs limited) and (3) the type of assurance practitioner (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 perceived reliability of the environmental and social information. There are no significant main effects for both the level of assurance and type of assurance practitioner. However, a significant interaction is found between these two experimental factors and report users' perceptions of reliability of such reports. More specifically, report users place more confidence in sustainability reports when the level of assurance provided is reasonable (that is, high but not absolute), and when such assurance is provided by a top tier accountancy firm, compared to when the assurance is provided by a specialist consultant. No such difference is found when the level of assurance provided is limited for either type of assurance practitioner group. The results of this study thus highlight the relevance of assurance for sustainability reporting.