Abstract: When the solution containing Mo(Ⅵ) is boiled in the presence of NH₂OH, Mo(Ⅵ) can react with 5-Br-PADAP, forming a water insoluble blue ternary complex Mo(Ⅵ)-NH₂OH-5-Br-PADAP. However, in the presence of OPthe complex does not precipitate. The molar ratio of the three components in the complex is estimated to be Mo(Ⅵ):NH₂OH:5-Br-PADAP =1:1:1. The wavelength of maximum absorbance for the complex is 605 nm. The molar absorptivity is 48000 at 605 nm. The calibration curve follows Beer's law in the range from o to 30μg of Mo (Ⅵ) in 10ml solution. As contrary to the usual practice masking agent is added after the colour development, and then the solution is boiled again to eliminate the effect of interfering ions. It is found that the method proposed in this work has very high selectivity.