Large corporations have traditionally prepared and verified non-financial information on a voluntary basis. In Spain, following the enactment of Law 11/2018, this practice became mandatory. These regulatory changes prompted an investigation into how prior voluntary practices were altered and whether a relationship exists between the auditing firm and the assurance provider. The research sample consists of companies listed on the IBEX 35 in 2018, analyzing both the first year of mandatory compliance under Law 11/2018 and the preceding year, 2017, when disclosure was voluntary. A database was constructed with the characteristics of non-financial information, enabling a descriptive analysis that segmented the sample by year, employed cross-tabulations, and applied chi-square tests. Big Four firms typically provide assurance for non-financial information while also auditing the financial statements of listed companies. Law 11/2018 did not significantly alter practices within IBEX 35 firms, as most had already adopted GRI (Global Reporting Initiative) standards voluntarily and continued reporting non-financial information in management reports once it became mandatory. Additionally, auditing and assurance providers show specialization according to industry sectors. Ongoing discussions about the future of non-financial information regulation in Europe highlight the relevance of contextualizing Spain’s starting point. Anticipated changes at the European level are expected to influence domestic regulation.
