Three dihydroxybenzene (DHB) isomers could not be simultaneously determined by ordinary spectrophotometry and fluorometric method because the overlaps of their absorption spectra and fluorescence spectra were severe. The total amount of these three isomers could be measured when they were synchronously scanned at Δλ＝0 nm because they have homoplastic character of fluorescence spectrum and favourable additivity of fluorescence intensity in 350～500 nm. It was found that when the three DHB isomers were bound with Cu^2＋ and isoniazide, ternary complexes were formed in a ratio of 1∶1∶2. When such complexes were synchronously scanned at Δλ＝30 nm using polarization and first order derivative techniques, the derivative fluorescence peak of m-DHB appeared at 260 nm, that of p-DHB appeared distinguishably at 320 nm, but that of o-DHB did not appear simultaneously at all. Thus the contents of m-DHB and p-DHB can be determined from their mixture respectively, and then the content of o-DHB can be obtained by subtraction of the contents of m-DHB and p-DHB from the total amount of the mixture. Hence these three DHB isomers can be simultaneously determined through the above method in the experiment. The system responds linearly to each of the three isomers concentrations in the range 3×10^－6～5×10^－4 mol/L with detection limit of 2.5×10^－7 mol/L for m-DHB, 3.6×10^－7 mol/L for p-DHB, the detection limit of the total amount of the mixture is 4.5×10^－7 mol/L, and the RSD of these systems was under 5%. The method is simple, convenient, quick, veracious and repeatable, which has been successfully applied to simultaneous determination of the three dihydroxybenzene isomers in water samples with satisfactory results.

Key words: synchronous-polarization-derivative fluorescence spectrometry, dihydroxybenzene isomer, isoniazide, Cu(II), simultaneous determination