Three side products (i) 7-chloro-1-ethyl-1 : 4 dihydro-4-oxoquinoline-3-carboxylic acid (IV), (ii) 7- chloro-4-ethoxyquinoline (VIII), and (iii) 7-chloro-1-ethyl-4 (1H)-quinolone (VI) have been isolated during preparation of 7 -chloro-4-hydroxyquinoline-3-carboxylic acid (I) from diethyl ethoxymethylenemalonate (EMME) and m-chloroaniline. The pyrolytic behaviour of the ester ethyl 7-chloro-4-hydroxyquinoline-3- carboxylate (II) and possibly the purity of EMME play an important role in their formation. Pyrolysis of the ester (II) in boiling diphenyl ether results in the formation of (a) ethyl 7-chloro-1- ethyl-1 : 4-dihydro-4-oxoquinoline-3-carboxylate (V), (b) 7-chloro-4-hydroxyquinoline (III), (c) (VI) and (d) (VIII) with evolution of carbon dioxide. A reaction mechanism consistent with the experimental data has been suggested.