Abstract:         We study how companies and stakeholders engage in social media communication to discuss Corporate Social Responsibility (CSR). We analyse the content of over a million microblogs on Twitter associated to the Spanish banking sector between November 2013 and February 2014. The content analysis is based on key issues considered by banking institutions in their CSR reports, which are the classified into Core and Supplementary CSR (Gomez-Carrasco et al., 2014 WP). We present evidence that CSR information has a relevant presence on social media. Second, our evidence suggests that internal users’ information is opportunistic as their microblogs are biased towards favourable information. Third, we show that information interests of internal and external users are significantly different. Core CSR information is mainly referred to by outsiders, whilst insiders mainly communicate Supplementary CSR information. Finally we provide descriptive evidence that firm size, listing status and impact on society are determinants for firm-initiated communication on social media.