A process for preparing tetracarboxylic acids of the general formula I or salts thereof, wherein at least one compound of the general formula II a or II b (a) is first reacted with at least one epoxidation reagent and (b) then oxidized with nitric acid or at least one nitrogen oxide, where the variables are defined as follows: R1, R2 are the same or different and are selected from hydrogen, phenyl and C1-C10-alkyl.