This study investigates the corporate environmental disclosure practices of companies on the Stock Exchange of Thailand in annual reports and websites. Content analysis by word count is used to determine the amount of disclosures on 50 sampled websites and annual reports. Statistical analysis (descriptive, paired samples T-tests, and independent samples T-tests), is employed to analyse the differences between environmental disclosures measured by word count in annual reports and on websites. Initial findings indicated that 96 percent of the sampled Thai listed companies provided environmental disclosures in their annual reports and 88 percent on websites. Companies providing the most words of environmental information were in the resources business group in both media; the smallest word counts were in the services business group on websites, but in the technology business group in annual reports. Commonly environmental disclosures were found in the corporate social responsibility section of the websites but in the corporate governance section of the annual reports. The three most common themes were environmental spending and activity, waste management, and environmental policy in both annual reports and websites. Statistically significant differences were found between high and low profile companies as well as government and private companies. However, this study could not find different amount of environmental disclosures made in annual report and on websites. The study has implications in enhancing the understanding the differences between disclosures in annual reports and on websites particularly in developing countries. The usual limitations associated with sampling, and content analysis apply. Disclosure quality was not considered.