NOVELTY - Biodiesel composition of improved oxidation stability and low acid number, comprises: major amount of biodiesel fuel consisting mixture of methyl or ethyl esters of fatty acids derived from fats and oils of vegetable or animal origin; and minor amount of substance or mixture of substances which is product of reaction between carboxylic acid compounds (I) and amine compounds (II). (I) is carboxylic acid and/or carboxylic ester. (II) is non-aromatic diamine or polyamine having at least one primary nitrogen atom separated by 2-3C atoms of another nitrogen atom which is primary and/or secondary.    USE - The composition is useful: in additive concentrate composition; and in diesel fuel composition (all claimed).    ADVANTAGE - The composition has improved oxidative stability and low acid number.    DETAILED DESCRIPTION - Biodiesel composition of improved oxidation stability and low acid number, comprises: major amount of biodiesel fuel suitable for use in compression ignition engine and consisting of a mixture of methyl or ethyl esters of fatty acids derived from fats and oils of vegetable or animal origin; and a minor amount of a substance or mixture of substances which is the product of chemical reaction between carboxylic acid compounds of formula (R1-C(=O)-O-R2) (I) and amine compounds of formula (H2N-CH(R3)-CH(R4)-(CH(R5))nNH(R6)) (II). (I) is carboxylic acid and/or carboxylic ester. (II) is non-aromatic diamine or polyamine having at least one primary nitrogen atom separated by 2-3C atoms of another nitrogen atom which is primary or secondary or their mixtures.    R1 = alkyl, hydroxyalkyl moiety of formula (CmH2m-x-k+1(OH)k), cyclohexyl or phenyl (optionally substituted by 1-12C alkyl);    R2 = H, CH3 or C2H5; either    R3, R4 = H or -CH3; or    R3R4 = cyclohexane;    R5 = H or -CH3;    R6 = H, alkyl moiety of formula (CjH2j-x+1) or amine moiety of formula (-CH2CH2(CH2)pNH(CH2CH2(CH2)pNH)qH);    j = 1-22;    m = 1-30;    x = 0, 2, 4 or 6 (preferably less than m);    k, n, p = 0 or 1; and    q = 0-4.    INDEPENDENT CLAIMS are also included for:    (1) an additive concentrate composition, comprising: a substance or mixture of substances (at least 5 wt.%) which is the product of the chemical reaction between (I) and (II); and a diluent (up to 95 wt.%) consisting of a liquid mixture of hydrocarbons having a flash point of at least 100 degrees C and a final boiling point of less than 340 degrees C, a biodiesel fuel or a blend of liquid hydrocarbons and biodiesel fuel;    (2) a diesel fuel composition, comprising: conventional diesel fuel derived from petroleum or other mineral fuels and suitable for use in a compression ignition engine ( less than or equal to 99 vol.%); and the biodiesel composition ( greater than or equal to 1 vol.%); and    (3) preparation of the substance or mixture of substances, comprising adding to the reaction mixture of a strong organic base, including tetramethylguanidine (0.1-5 wt.% based on weight of (II)).