In 1982, a collaborative test was conducted at a scrubber-equipped, coal-burning power plant to determine the precision, accuracy, and reliability of Method 6B, an Environmental Protection Agency (EPA) test method for analyzing sulfur dioxide and carbon dioxide and for determining the sulfur dioxide emission rate. The quality assurance practices used in the collaborative test are discussed. An important part of the test was preliminary practice runs to allow participants to learn the new procedure. During the test, 24-h samples were collected over a 5-day period by Method 6B, using both continuous and intermittent sampling procedures. In addition, samples were collected for reference data using EPA Methods 6 and 3 and using the plant's continuous emission monitor. The analysis for relative accuracy indicated that the results by Method 6B for both sulfur dioxide and carbon dioxide were within 3% of those obtained by independent methods (i.e., Methods 6 and 3 and the continuous emission monitor). The analysis for precision indicated that repeatability and reproducibility of the sulfur dioxide emission rate were 8 and 11.1%, respectively.