Abstract: A new catalytic spectrophotometric determination of trace manganese has been developed based on catalytic oxidizing discoloration of eriochrome black T with hydrogen peroxide using sodium dodecylbenzene sulfonate as sensitizer in Na₂B₄O₇-NaOH buffer solution at pH 11.0. The Beer's law is obeyed in the range of 5.0～12.0 μg/L, the linear regression equation is △A=-0.014+0.032C_Mn(Ⅱ)(μg/L), r=0.999 9, the detection limit for manganese is 0.5 ng/mL. The method has been applied to the determination of Mn(Ⅱ) in the hair and tea samples with satisfactory results.