This study compares the Internet (corporate web pages) and annual reports as media of social responsibility disclosure (SRD) and analyses what influences disclosure. It examines SRD on the Internet by Portuguese listed companies in 2004 and compares the Internet and 2003 annual reports as disclosure media. The results are interpreted through the lens of a multi-theoretical framework. According to the framework adopted, companies disclose social responsibility information to present a socially responsible image so that they can legitimise their behaviours to their stakeholder groups and influence the external perception of reputation. Results suggest that a theoretical framework combining legitimacy theory and a resource-based perspective provides an explanatory basis for SRD by Portuguese listed companies.