This project was conducted to evaluate a number of commercially available extractive-type sampling and monitoring systems for monitoring sulfur dioxide and hydrogen sulfide source emissions. Evaluation testing was performed at a Fossil Fuel-Fired Power Plant and at a Claus Sulfur Recovery Plant to obtain representative ranges of stack gas temperature, water and particulate loading, and corrosiveness. Tests of calibration error, relative accuracy, two- and twenty-four-hour zero and calibration drifts, response time, and operational period were performed in accordance with published EPA guidelines. The detailed field test results, the complete Work Plan, sampling interface drawings, the results of evaluations of Compliance Test Methods 6 and 11, miscellaneous observations and results, and a discussion of the calculation and reporting instructions of the EPA guidelines are given in the appendices. A detailed description of the systems evaluated, summaries of the field test results, and relevant comments concerning the results are given in the body of the report. Because the sites chosen for evaluation testing provided wide ranges of sample temperature, solids and water loading, and SO2 concentration, the results are considered to be relevant to most stack-gas monitoring problems.