3, 4-Dinitrophthalic acid gave a sensitive violet coloration with glucose when heated in aqueous sodium carbonate. In this color reaction, the acid was reduced to 3-nitro-4-hydroxylaminophthalic acid. 4, 5-Dinitrophthalic acid behaved in the same way as the 3, 4-isomer, and was reduced to 4-hydroxylamino-5-nitrophthalic acid. 3, 5-Dinitrophthalic acid was less sensitive in the reaction, showing a yellow coloration by its reduced substances down to 3-amino-5-nitrophthalic acid.