The concurrent formation of furan-2,5- and furan-2,4-dicarboxylic acid under solvent free conditions via a disproportionation reaction is described. By reacting potassium-2-furoate at 260 degrees C in the presence of 22 mol% of (Lewis acidic) catalysts like CdI2 or ZnCl2, potassium-2-furoate is disproportionated to furan and furandicarboxylic acids. Besides furan and furan-2,5-dicarboxylic acid (2,5-FDCA) as the main products, furan-2,4-dicarboxylic acid (2,4-FDCA) is also formed as a by-product. Experimental evidence has been obtained that, under the reaction conditions applied, 2,5-FDCA and 2,4-FDCA are formed by separate reaction pathways. Selectivity towards the different FDCA isomers is affected by the type of catalyst used. Single-crystal X-ray analysis shows that 2,4-FDCA has a more 'linear' character compared to 2,5-FDCA and hence is structurally more comparable to terephthalic acid (TA), making it an interesting monomer for synthetic polyesters.