Assurance of sustainability reports has experienced a growing trend at an international level in the last decades, promoting the creation of a market whose knowledge is still limited. The aim of this paper is to investigate the market for assurance of sustainability reports in Spain from the demand perspective. We identify not only the characteristics of the companies that hire this service, but also their strategy and attitude towards assurance. We conduct a survey within a sample of 342 companies that have registered some of their sustainability reports in the Global Reporting Initiative (GRI) database.

Our results reveal that large companies, operating in sectors with greater environmental sensitivity and listed on the secondary market, demand the external assurance service to a larger extent. In addition, the study of the company's characteristics that determine the demand for assurance points out differences in the stakeholders pressures as the main determining factor. However, when companies decide to assure their sustainability reports, they follow a similar behavior, in order to increase credibility and reliance in sustainability information, especially those with a greater social and environmental impact.