A method for the production of a keto acid having the general formula ##STR1## wherein R1 and R2 independently represent a straight or branched chain alkyl of 1-18 carbon atoms, a cycloalkyl of 4-8 carbon atoms or a phenyl, each of which may be substituted by at least one substituent selected from the group consisting of halogen atoms and alkyls having 1-4 carbon atoms, an aralkyl of 7-10 carbon atoms, alkoxyalkyl having 2-20 carbon atoms, tetrahydrofuryl alkyl, alkylcarboxy alkyl, alkylcarboxy benzyl, or R1 and R2 together with the adjacent nitrogen atom may form a heterocyclic ring, or one of R.sub.1 and R.sub.2 is hydrogen, but R1 and R2 may not simultaneously be methyl or ethyl or benzyl which comprises reacting a m-amino phenol having the general formula ##STR2## wherein R1 and R2 are the same as above, with phthalic anhydride, in an organic solvent in an amount of less than 0.5 parts by weight per part of m-amino phenol.