A urea SCR deNOx system for heavy-duty diesel engines was developed comprising a SCR catalytic converter combined with a low sulfate formation oxidation catalyst. Urea is injected upstream the SCR catalyst system by means of an open-loop controlled injection system.Test bed investigations with the urea deNOx system applied to a 315 kW EURO 2 baseline heavy-duty diesel engine demonstrated a NOx emission reduction potential down to a level of less than 3 g/kWh. Heavy-duty diesel engines equipped with such a deNOx converter are able to comply with the expected EURO 4 NOx emission limits, independent from the ultimately effective test cycle. The particulate emission level of the EURO 2 engine was found not to be influenced by the SCR catalyst. The combination of SCR and oxidation catalyst results in a particulate increase of up to 25%, depending on the utilized test cycle.Over the FIGE transient cycle the influence of the oxidation catalyst stage on ammonia and N2O emissions was investigated. It was found that a reduction of the already low ammonia slip after the SCR stage down to a level of less than 3 ppm is achieved. However, the N2O emission increases over the oxidation catalyst to levels comparable to current passenger cars. Besides that, CO, HC including unregulated emissions, and irritating exhaust odor are considerably reduced.