Abstract: The ultraviolet spectrophotometry based on Kalman filtering(KF) was applied to determine simultaneously three isomeric dihydroxybenzenes,catechol(1,2-dihydroxybenzene), resorcinol(1,3-dihydroxybenzene and hydroquinone(1,4-dihydroxybenzene) without preliminary separation.The ultraviolet absorption peaks of their aqueous solutions (pH=3.6) are 275nm,273nm and 288mp respectively.The average recoveries of catechol, resorcinol and hydroquinone in standard mixtures are 97.4%,101.7% and 98.0%,with standard deviations of 1.72%,3.86% and 2.04% (n=10),respectively.The proposed method has been used for the simultaneous determination of the thine isomers in simulating samples with satisfactory results.