The U.S. Environmental Protection Agency has the responsibility to assess the accuracy of the air monitoring data submitted to it by the state and local agencies throughout the United States. This task is accomplished through audits administered by the National Performance Audit Program. This paper discusses a device utilized by this program to provide accurate sulfur dioxide concentrations suitable for auditing the accuracy of continuous, ambient SO2 analyzers. Designed to be mailed to the audited agency, this compact, lightweight device has proven to be a reliable and accurate SO2 audit system. The development, evaluation, acceptance tests, and five years of field audit data are presented.