In response to investor and public concerns about harm to the environment, companies are increasingly disclosing environmental information. To enhance the quality of corporate environmental disclosures in a largely voluntary context, various stakeholders are also demanding independent assurance. This study uses a stakeholder-agency theoretical perspective to examine whether the quality of voluntary environmental disclosures is enhanced when assured. This study also examines the difference in the quality of voluntary environmental disclosures when assurance is provided by either professional accountant assurers or private consultants. Our sample comprises listed companies voluntarily disclosing environmental information in stand-alone reports, annual reports, and websites. We use an index based upon the Global Reporting Initiative (GRI) to measure the quality of company environmental reporting. Results of this matched pairs study show that the quality of voluntary environmental disclosures scores significantly higher for assured companies than unassured companies. For assured companies, the quality is no different when assured by accountants or consultants. Additional analysis provides preliminary evidence that experience improves the quality of environmental disclosures.