Spain is the world's leading country as regards corporate social responsibility (CSR) reporting (KPMG, 2011). In addition, Spain is taking new initiatives with regard to environmental policy, sustainable development, and stakeholder engagement in accordance with Law 2/2011.This is why we choose Spain as the setting to analyze whether the determinants for external assurance posited by existing literature (industry, size, profitability, leverage) have an impact on the decision of companies to assure their CSR reports. Our study is a pioneer in the sense that it investigates the possible links between the auditor of the annual report and the provider of assurance. The results of this study indicate that assurance of CSR reporting depends on the size, leverage, and profitability of the company. We find that the sustainability assurance market is dominated by the Big‐4 firms, who develop different strategies in this field. Finally, we find that the decision to hire an auditor to perform sustainability assurance sometimes depends on the industry. Copyright © 2012 John Wiley & Sons, Ltd and ERP Environment.