A review of the type and nature of the various performance tests used by the United States Environmental Protection Agency (EPA) to certify commercially available air monitoring instruments. Examples of test results for preand postcertification performance tests and for EPA simultaneous comparison tests for several equivalent methods for sulfur dioxide are also described. Forty-seven methods of ambient sulfur dioxide detection, which include thirty-nine different instrument models, have been certified. A list of commercially available, certified sulfur dioxide analyzers is given, which will aid conservation scientists and conservators in choosing appropriate ambient air monitors for museum and laboratory spaces. -- AATA