Purpose This paper aims to examine the practice of sustainability assurance and in particular the content of the assurance reports disseminated in the corporate social responsibility (CSR) reports. The objective of the research is to study the evolution of the content of the assurance reports of French companies. Have the reports evolved as a result of the standardization and regulation of the audit? Design/methodology/approach A longitudinal study has been carried out on French companies publishing CSR reports and their sustainability information has been checked for several years. The sample is composed of 19 listed French companies and 135 assurance reports over a period from 2001 to 2015. Findings The results highlight a change in the content of assurance reports according to standardization and regulatory of sustainability auditing. The content evolves from a generic discourse to a normative discourse notably because of the realization of a majority of assurance missions by accounting professionals and the increasing use of ISAE 3000 as well as by the introduction of the Grenelle II Law. Practical implications This paper shows how assurance reports have evolved over time in a particular regulatory context of the introduction of a law specific to assurance, Grenelle II Law, using a sample of French companies. Originality/value Although some studies have attempted to provide a historical analysis of this practice, no research has focused on longitudinal analysis in a particular context of introduction of a law specific to the assurance mission. The lack in previous literature resides in the lack of longitudinal analysis of assurance reports in the light of the evolution of the normative and regulatory frameworks.