The Global Reporting Initiative (GRI) can be considered as one of the most extensively used model of corporate social responsibility (CSR) reporting. The assurance statement (AS) is the mechanism to give trust to stakeholders about the accuracy of the information published in the sustainability reports (SRs). Within this backdrop, the objective of this paper is to design a model to evaluate the quality of the SR and AS of the Spanish companies that present SR using the GRI guide for the year 2008, as well as the characteristics of the companies that present high quality reports. We find that auditors issue higher quality AS than consultants, and that measures indicating size (size, listed companies, Big4 auditor) are associated with the quality of AS reports. Finally, we find that variables related to the industry of the company (sector, size, leverage) are associated with the quality of the SR report.