In recent years, sustainability reports have been gradually adopted by businesses and organizations in order to enhance their preparedness, competitiveness, and ability to adapt to a rapidly changing world. The aim of this study was to assess the comprehensiveness of 44 Greek companies' corporate social responsibility (CSR) reports published in 2016. For this purpose, an evaluation methodology was developed in line with the accountability reporting principles suggested by the Global Reporting Initiative (GRI)-G4 guidelines for preparing and publishing sustainability reports. The overall findings indicate moderate levels of satisfaction as the sample Greek firms performed adequately at embedding the GRI reporting principles into their corporate disclosures. Most firms' integration of the "accuracy" and "reliability" principles was poor. Emerging from the latter outcome, together with the low integration of some materiality issues, are problems of transparency, credibility, balance, and completeness related to the publishing of CSR reports. It is likely that operational features such as type, size, and sector have the potential to influence the style and comprehensiveness of reports.