This paper consists in a longitudinal study of corporate social disclosure (CSD) practices of Pechiney during the second part of the XXth century. The research aims at identifying the determinants of such practices. The objective is to understand the motivations of companies in disclosing non financial information. Our theoretical framework is derived from legitimacy theory. We argue that CSD can be considered as a legitimation strategy and we document the link between CSD practices and public’s concern.. Based on media agenda-setting theory, we suggest a measure of public pressure. Our results show that CSD is a means used by firms to inform its stakeholders and also to manage public impressions.