TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The aromatic diamine is a compound of formula (I); N-phenyl-N'-alkyl-para-phenylene diamine; or is a blend of N-(1,4-dimethyl-butyl)-N'-phenyl-1,4-phenylenediamine and N-(1,3-dimethyl-butyl)-N'-phenyl-1,4-phenylene diamine) and the sterically hindered phenol is a compound of formula (II) or 2,6-di-tert-butyl-4-methyl phenol and/or 1,2,3-trihydroxy-benzene.R1 and R2=alkyl, phenyl, sec-butyl, isopropyl, 1,4-dimethyl butyl, 1,3-dimethyl butyl and/or 1,4-dimethyl pentyl;R3-R5=alkyl, H, tert-butyl and/or methyl-phenyl.Preferred Mixture: The mixture (M1) has a ratio of aromatic amine to hindered phenol of 1:9 to 9:1 (preferably 1:4 to 4:1, especially 1 to 1) wt.%. The mixture is present in the biodiesel fuel in an amount of 1-10000 (preferably 50-5000, especially 100-2000) parts per million. The mixture (M1) further comprises polyhydroxy phenolic compound of formula (III).R=alkyl, H, COOH or COOR6;R6=ester.Preferred Composition: The composition further comprises at least one other component selected from dispersant/detergents, octane improvers, cetane improvers, carrier fluids, demulsifiers, antifoam agents, anti-icing additives, biocides, combustion improvers, alkali or alkaline-earth metal detergents, drag reducers, metal deactivators, lubricity additives, dyes, markers, odor masks, odorants and stability improvers. EXAMPLE - A 1:1 wt.% mixture of N-420 (RTM: N-phenyl-N'-alkyl-para-phenylene diamine) and Pyrogallol (RTM: 1,2,3-trihydroxy-benzene) was added to rapeseed-ME i.e. methyl ester of rapeseed oil biodiesel at a treat rate of 1000 parts per million (wt./wt.) to prepare a composition (test). A control was prepared by adding N-420 to rapeseed-ME biodiesel at treat rate of 1000 ppm (wt./wt.). The antioxidancy of control and test was measured using Rancimat EN 14112 Oxidation Test. The antioxidancy was measured in terms of oxidation induction time (OIT) at 110degreesC. The test composition displayed a synergistic oxidative response and had an induction time of 16.5 hours which was 83.3% and negative 3% increase over control that showed induction time of 9 hours. The results showed that the test having synergistic antioxidant mixture had increased oxidation induction time as compared to control containing N-420 alone.