The invention is directed to a process for preparing maleic acid or a derivative thereof, the process comprising a step b) of oxidizing 5-hydroxy-2(5H)-furanone and/or cis-β-formylacrylic acid to maleic acid or a derivative thereof by contacting the 5-hydroxy-2(5H)-furanone and/or cis-β-formylacrylic acid with molecular oxygen (O2) in the presence of a catalyst. In a particular embodiment, the step b) is preceded by a step a) of oxidizing a furanic compound according to formula I into the 5-hydroxy-2(5H)-furanone and/or cis-β-formylacrylic acid,




wherein R1 is H, CH2OH, CO2H or CHO and R2 is H, OH, C1-C6 alkyl or O(C1-C6 alkyl), or esters, ethers, amides, acid halides, anhydrides, carboximidates, nitriles, and salts of formula I.