The aim of this paper is to explore the relevance of institutional influences on corporate environmental reporting practices. The wind energy sector in Spain was chosen for analysis because of its rapid growth and significant impact on moves towards cleaner energy production. A content analysis of recent sustainability reports (2005–2009) from seven main wind energy companies facilitated a longitudinal comparison of the levels of compliance with Global Reporting Initiative indicators of sustainability. Results show that initial institutional pressures for reporting have been replaced by imitation by companies of each other's practices leading to a reduction in the differences between environmental disclosures. Results confirm the importance of examining different institutional pressures on disclosure practices to development of policy. However, a key finding is that the disclosures have been minimal and their lack of effectiveness and unreliability mean they are not conducive to the notion of increasing transparency in the wind generated clean energy electricity sector. As a result the credibility of relying on a voluntary sustainability standard, such as the Global Reporting Initiative (GRI), as an incentive for informative environmental reporting is challenged.