A new tool for SCR (selective catalytic reduction) testing is developed. This side stream test bench has an advantage to test smaller catalysts with a proper exhaust. The exhaust mass flow and temperature are adjustable. A control program is developed to adjust and save the needed parameters. NOx sensors are utilized in the test bench to measure the levels upstream and downstream of SCR reactor. In this study one SCR with a volume of 40.5 dm3 is tested with exhaust flows ranging from 400 to 600 kg/h and exhaust temperatures from 280 to 360°C. The tests are conducted using exhaust gas from a medium-speed diesel engine running on heavy fuel oil. A decrease from 75% to 99% (depending on test conditions) was observed in NOx over the catalyst. In addition HC and PM were found to decrease while CO was increased over the catalyst. The controlling of exhaust flow and temperature with the test bench succeeded with only minor errors. The measured value of exhaust flow deviated from the set value in the order of 1% and temperature in the order of 0.1%. The results of NOx sensors correlated well with the standardized NOx measurement results with an exception being the case when ammonia slip was formed. When ammonia was found in the exhaust the NOx sensors were observed to misleadingly measure the ammonia as NO. The results with different exhaust flow rates revealed that SCR was performing better when the flow was lower, i.e., the space velocity was lower. At higher temperatures higher NOx conversions were observed. At 360°C the NOx conversion was near 95% while at 280°C the conversion was 75%.