The principle of the method is preparing a cyclohexane solution of the sample to be analyzed, measuring the absorbance for the 810 cm<(hoch)-1> and 782 cm<(hoch)-1> bands which are characteristic of the 1,2,4- and 1,2,6-positions on the aromatic ring, and calculating the absorbance ratio. Applies to mixtures containing 5 % to 95 % of the 2,4-isomer.