Abstract: In a dilute sulfuric acid solution,a remarkable induced catalytic effect of vitamin C on the oxidative decoloration of dimethyl yellow by H2O2 was observed,and the same oxidative decoloration reaction was further catalyzed by the presence of hexavalent chromium.Linear relationship between the magnitude of decrease in absorbance at 520 nm and the concentration of Cr(Ⅵ) was found to be kept in the range of 0.0-0.10 μg per 25 mL of solution,with a detection limit of 8.14×10-8g·L-1 of Cr(Ⅵ).Through the study for the optimum conditions of the decoloration photometric reaction and for the kinetic parameters,a new and sensitive method for the photometric determination of Cr(Ⅵ) was proposed,and in its application to the analysis of fruits,leaves,and branches of oranges and orange trees,the results of RSD′s were less than 5.6% and recoveries found were in the range of 98.7%-103.1%.