Abstract: It was found that the oxidative decoloration of methyl thymol blue (MTB) by potassium periodate was catalyzed by the presence of trace amounts of palladium(Ⅱ),and that a linear relationship between magnitude of decrease in absorbance of the reaction system and concentration of palladium was obtained in the range within 85 μg·L-1,with detection limit (S/N=3) of 5.23×10-6g·L-1.Base on these facts,a new catalytic spectrophotometric method for determination of Pd(Ⅱ) in trace amounts was proposed.Measurements of absorbance were made at the wavelength of 441 nm,which is the absorption maximum of methyl thymol blue under the selected reaction condition.Apparent activation energy of this catalytic reaction is found to be 60.03 kJ·mol-1.The proposed method has been applied to the determination of palladium in samples of palladium containing catalyt,giving values of RSD′s (n=5) and recovery in the ranges of 1.54%-2.01% and 97.8%-100.8% respectively.