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Metal oxides comprise a large group of chemicals used in water treatment to adsorb organic pollutants. The ability of titanium dioxide (TiO2) and iron (III) oxide (Fe2O3) to reduce the chronic toxicity of (phenolic) C6H6(OH)2 isomers, namely hydroquinone (HQ) and catechol (CAT) to Ceriodaphnia dubia and Pimephales promelas (less than 24 h-old) were investigated. The toxic endpoints following metal oxide treatment were compared to endpoints of untreated CAT and HQ. In chronic toxicity testing, HQ resulted in greater toxicity than CAT for both test organisms; the median lethal concentrations (LC50) for CAT were 3.66 to 12.36 mg.L−1 for C. dubia and P. promelas, respectively, while LC50 for HQ were 0.07 to 0.05 mg.L−1, respectively. Although both treated solutions presented lower toxic endpoints than those in the untreated solutions, Fe2O3 had a better potential to reduce the toxic effects of CAT and HQ than TiO2. [ABSTRACT FROM AUTHOR]