TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Compounds: The antioxidant (A1) comprises a reversibly redox-able organic mediator (preferably a hydrocarbon system compound composed of four elements: C, O, N, and H. The antioxidant: (i) has a reversible redox-ability, and an oxidant of the antioxidant is chemically stable; (ii) is changeable to a chemically stable hydrolysate; (iii) has a standard oxidation-reduction potential greater than 0.68 V (normal hydrogen electrode (NHE)) and smaller than 2.85 V (NHE) (preferably greater than 0.68 to less than 1) V (NHE); and (iv) has a concentration of 10-4 - 1 (preferably 10-3 - 1)% to the fuel. The antioxidant comprises amine compound of formula (I), imide compounds of formulae (II), (III), (IVa), (IVc), (IVd), (IVf), or (V); or tricyclic imide of formula (VIa) or (VIb); or amine compounds of formula (VII) or (VIII); or imide compounds of formulae (IX) - (XI). The imide compounds of formulae (IVa), (IVc), (IVd), or (IVf) are selected from N-hydroxy succinic acid imide, N-hydroxy maleic acid imide, N-hydroxy hexahydrophthalic acid imide, N,N'-dihydroxycyclohexane tetracarboxylic acid imide, N-hydroxyphthalimide, N-hydroxy tetrabromophthalic acid imide, N-hydroxy tetrachlorophthalic acid imide, N-hydroxy fatty acid imide, N-hydroxy himic acid imide, N-hydroxy trimellitic acid imide, N,N'-dihydroxy pyromellitic acid imide, or N,N'-dihydroxynaphthalene tetracarboxylic acid imide. The imide compounds of formulae (V), (VIa) and (VIb) are selected from N-hydroxyglutaric acid imide, N-hydroxy-1,8-naphthalene dicarboxylic acid imide, N-hydroxy-1,8-decalin dicarboxylic acid imide, N,N'-dihydroxy-1,8:4,5-naphthalene tetracarboxylic acid imide, N,N'-dihydroxy-1,8:4,5-decalin tetracarboxylic acid imide, or N,N',N''-trihydroxy-isocyanuric acid imide.R1 and R2=substituent groups;X=O or OH;a=optional bond;Y1=5- to 12-membered ring (optionally containing double bonds or optionally aromatic);R3 and R4=H, halo, alkyl, aryl, cycloalkyl, OH, alkoxy, carboxyl, alkbxycarbonyl or acyl;n=1 - 3;R3+R4=double bond, or optionally aromatic 5- to 12-membered ring (preferably cycloalkane, a cycloalkene, a bridged hydrocarbon ring, or aromatic ring (all optionally substituted));R3a, R4a, R5 and R6=H, halo, alkyl, OH, alkoxy, carboxyl, alkoxycarbonyl, acyl, nitro, cyano, or amino;R7 - R12=H, alkyl, aryl, cycloalkyl, OH, alkoxy, carboxyl, carbonyl, acyl, or acyloxy;R13 - R18=H, alkyl, OH, alkoxy, carboxyl, alkoxycarbonyl, acyl, nitro, cyano, or amino;R19 - R24=alkyl (optionally substituted);R20+R21 and R23+R24=a ring;Y2=5- or 6-membered ring;Z=H, alkyl, aryl, alkoxy, carboxyl, alkoxycarbonyl, cyano, OH, nitro, or amino (preferably alkyl (chained, ringed or branched, and containing O and N), or aryl (optionally substituted and containing O and N)).The aromatic or non-aromatic ring may be formed by at least two of R7 to R12 combined with each other comprises an N-substituent cyclic imide group; at least one of R19 - R21 is chained, ringed, or branched; at least two of R19 to R21 are mutually combined to form a ring; and at least one of R19 - R24 includes O and N atoms. EXAMPLE - A fuel cell system was prepared. A film of Nafion 117 (RTM: solid polymer electrolyte membrane) was boiled in 3% aqueous H2O2 solution for 1 hour and in distilled water for 1 hour, in sulfuric acid (1 M) for 1 hour, and finally in water for 1 hour. The membranes were subjected to ion exchange treatment with FeSO4 solution (100 mM). The ion-exchanged electrolyte membrane was coated with platinum-ruthenium-supporting carbon (catalyst) to fabricate membrane electrode assembly (MEA). Fabricated MEA was assembled in a single cell to provide a unit cell. N-hydroxyphthalic acid imide (NHPI) (50 ppm) was dissolved as antioxidant to aqueous solution of methanol (10%) to obtain a solution as a fuel. The cell temperatue was 70degreesC. A fuel cell prepared similarly but using a fuel without containing antioxidant was used as a (control). The cells were evaluated for redox potential and start stop endurance test (where the operation was repeated, and the number of times of repetition were counted). It was observed that the redox potential and repetition time number were as follows: 1.34 V and 300, respectively for the (test) cell; and the (control) cell had repetition time number was 80.