The paper discusses the use of EPA's bench-scale pilot plant to evaluate catalysts used in the ammonia (NH3)-based technology and process for selective catalytic reduction (SCR) of nitrogen oxides. A key objective was to establish the performance of SCR catalysts on U.S. fuels and combustion sources. A rudimentary catalyst produced inhouse and two commercial catalysts were evaluated. The temperature ranged from 327 to 440 C. The space velocity ranged from 7650 to 36,500 hr to the -1 power. The combustion gas was doped with nitric oxide (NO) and NH3, and the NH3/NO ratio ranged from about 0.6 to 2.2. Sulfur dioxide (SO2) was added to the combustion gas in some runs to investigate its effect on NO conversion over one commercial catalyst. The formation of nitrous oxide (N2O) on the same catalyst at 400 C was also investigated. The evaluation indicated that, for the inhouse catalyst, the space velocity has a significant effect on NO conversion. For the two commercial catalysts, the NO conversion was 90% and higher when the NH3/NO ratio was near or above unity, and the NO conversion was approximately proportional to the NH3 concentration at the inlet of the reactor when the ratio was less than unity. Flue gas SO2 was found to be poisonous to one commercial catalyst. Furthermore, the amount of N2O formed over the same commercial catalysts was negligible.