Abstract: The present paper contains the A～t kinetics activity that exhibits in the process of reaction of Cr(Ⅲ) and Fe(Ⅲ) ions with XOand other colorants.Two complex compounds derived from the reaction of Fe (Ⅲ) and Cr (Ⅲ) with xylenol orange and other colorants have similar light absorbance but their speeds of reaction are obviously different while the repeatability of their A～t curves is satisfactory accordingly, a new differential speed kinetics analysis has been devel-opped, with which the bimixtrue of Fe(Ⅲ) and Cr(Ⅲ) was determined simultaneously.The result is satisfactory when it is used in the determination of sample sewage.Linearity 0～0.25 mg/ mL, recovery 95%～106% and detection limit 0.1μg/mL.

Key words: Xylenol orange, Cr(Ⅲ), Fe(Ⅲ), Differential speed kinetics