Tax Exempt Organizations - Form 990

http://www.irs.gov/charities/article/0,,id=139025,00.html

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"A tax-exempt organization is generally not required to disclose the names or addresses of its contributors on its annual return, including Schedule B (Form 990, 990-EZ, or 990-PF). The regulations specifically exclude the name and address of any contributor to the organization from the definition of disclosable documents. Contributor names and addresses listed on an exempt organization's exemption application are subject to disclosure, however.

This general exclusion for contributor information on annual returns does not apply to private foundations, or to political organizations described in section 527. Certain tax-exempt political organizations are required to report the name and address, and the occupation and employer (if an individual), of any person that contributes in the aggregate $200 or more in a calendar year on the Schedule A of Form 8872. Tax-exempt political organizations may also be required to file Form 990, including Schedule B. Political organizations are required to make both of these forms available to the public, including the contributor information."
http://www.irs.gov/charities/article/0,,id=135015,00.html