There is a rising trend among companies to publish their sustainability or corporate social responsibility (CSR) reports. Assurance of these reports is a valuable voluntary tool to provide them with higher credibility. Nonetheless, the quality of assurance reports differs in practice and the objective of this paper is to provide evidence in this new area of research. Indeed, we are pioneers in developing an index to measure the quality of assurance reports. We choose the Spanish setting because it is the worldwide leading country as regards CSR reporting (KPMG, ; Sierra et al., ). We have found evidence on the determinants for CSR reporting posited by existing literature that have an impact on (a) the decision of companies to publish their CSR reports, (b) the decision to assure the CSR report or not and (c) the decision to hire the assurance services from an auditor or a consultant and the subsequent quality of the assurance report. Last but not least, our results from a sample of 161 CSR assurance reports evidence that assurance reports are of fairly acceptable quality, according to the index proposed. Furthermore, the value of the quality index is significantly higher if the assurance services are provided by an auditor (as opposed to a consultant) and if the CSR reporting company is larger. Copyright © 2012 John Wiley & Sons, Ltd and ERP Environment.