Background:           Corporate social responsibility has become a greater concern over the past decades. Today it is a common practice for companies to issue sustainability reports to inform on their sustainable behaviour. The Global Reporting Initiative has been on the forefront of illuminating the issue of sustainability and providing a standardised framework for sustainability reports.

Purpose:                  The objectives of this research are to investigate factors for adoption of GRI-based sustainability reports and inclusion of external assurance statements, as well as if those are related to an additional market premium for firms listed on the Nasdaq OMX Stockholm.

Method:                  This paper examines 181 companies listed on the Nasdaq OMX Stockholm, listed for the entire period 2014-2016. Four models are constructed to test hypothesis connected with the purpose.

Conclusion:    Companies using GRI-based reports have a positive association with the market to book valuation. Legitimacy theory provide us with an answer why companies produce GRI-based reports. In addition, both signalling and legitimacy theory are used to explain the extra market premium associated with GRI-reporting.