It is an overwhelming argument that the use of biodiesel instead of petrodiesel causes a reduction in harmful exhaust emissions from engines. A number of studies, however, indicate substantial increases in engine out NOx emissions with biodiesel fuel. Some studies have pointed out that the increased formation of prompt NOx is responsible for biodiesel NOx effect. Treatment of biodiesel with antioxidants is a promising approach because it reduces the formation of hydrocarbon free radicals, which are responsible for prompt NOx production in combustion process. Aromatic amine antioxidants are known as to be efficient inhibitors of free radicals. This study examines the use of p-phenylenediamine derived aromatic amine antioxidants for NOx reduction in a soybean biodiesel fuelled DI diesel engine. The antioxidant additives, N,N'-diphenyl-1,4-phenylenediamine (DPPD) and N-phenyl-1,4-phenylenediamine (NPPD) were tested on a computerised Kirloskar-make 4 stroke water cooled single cylinder diesel engine of 4.4 kW rated power. Results show that significant reduction of NOx could be achieved by the addition of antioxidants but smoke, CO and HC emissions were found to have increased. (C) 2012 Elsevier B.V. All rights reserved.