Heartland pushes for deregulation and is especially known for its opposition to scientists who have concluded human activity is causing global warming.
The documents appear to show large donations from the liquor and tobacco industries, the General Motors Foundation and an anonymous donor who contributed $14 million, The Guardian reports.
At the same time, John Mashey, a retired computer scientist in California, has filed a complaint with the Internal Revenue Service charging Heartland has violated its tax-exempt status. Mashey told the Guardian he spent three months studying Heartland's finances.
The Heartland document dump appears to be a response to "Climategate," the publication of internal e-mails at a University of East Anglia institute studying global warming. A group of climate scientists wrote Heartland asking the institute to "recognize how its attacks on science and scientists have poisoned the debate about climate change policy."
The Heartland documents and Mashey's findings were published on the Web site DeSmogBlog, The Christian Science Monitor reported. They included the 2012 budget, a fundraising plan and board minutes.
Heartland charges that one document is a forgery.