The report presents information obtained in the evaluation and collaborative testing of a reference method for measuring the sulfur dioxide content of the atmosphere. The technique is called the pararosaniline dye method or sometimes the West-Gaeke method. Different variations of this method have been used extensively by many laboratories since the original publication in 1956, and it has been found to be reliable and reasonably free of interferences. Collaborative tests were performed involving a total of eighteen laboratories. A statistical analysis of the data of fourteen laboratories provided the following results, based on the analysis of pure synthetic atmospheres using the 30-min sampling procedure and the sulfite calibration method prescribed. Results are also presented with respect to the use of control samples and reagent blank samples, the minimum number of samples required to establish validity of results within stated limits, and the statistical evaluation of various steps included in the method. The method can give satisfactory results only when followed rigorously by experienced laboratory personnel. The publication of the method in the Federal Register, April 30, 1971, as the reference method to be used in connection with Federal ambient air quality standards for sulfur dioxide is appended.