The U.S. Environmental Protection Agency designates specific ambient monitoring methods and analyzers as reference or equivalent methods acceptable for use in certain required monitoring. Such designation is based on test of the performance of the analyzer model by the manufacturer (or other applicant) prior to designation. After designation, EPA carries out further tests of the analyzer, including performance tests as well as comparative tests to evaluate the performance, reliability, and operational peculiarities of the analyzer with respect to other analyzers monitoring the same pollutant. This report summarizes both the manufacturer's predesignation test results and the available EPA postdesignation test results for currently designated sulfur dioxide analyzers. Manufacturer's predesignation test results are presented for thirteen analyzers, and all test results met or exceeded the specifications. EPA postdesignation performance test results are presented for ten analyzers, and similarly, all test results met or exceeded the specifications. EPA postdesignation comparative test results are presented for eight analyzers, with the results indicating that the analyzers tested were generally stable and reliable and demonstrated a high degree of comparability.