Abstract: In this paper the utilization of ¹H NMR including both peak height and integration determination methods is reported in the estimation of the relative contents of ethyl 7-chloro-l-ethyl-6-fluoro-1.4-dihydro-4-oxoquinoline-3-carboxyla-te and its 5-chloro isomer which were produced in the cyclization reaction of di-ethyl 3-chloro-4-fluoroanilinemethylene-malonate in the presence of different Lewis acids.