The synthetic procedure of the titled reagent and its colour reaction with U (Ⅵ) is reported. The reagent was obtained by the coupling reaction of m-nitroanilinediazonium salt with bromophosphonazo I. It formed a stable blue-purple complex with U (Ⅵ) in a strong acidic medium and the wavelength of maximum absorbance was observed at 684nm. Beer's law was obeyed over the range of 0—45

µ

g U (Ⅵ) in 25 ml. The value of molar absorptivity at 684 nm was found to be 8.4×10

4

1•mol

-1

•cm

-1

and 1.0×10

5

1•mol

-1

•cm

-1

in the presence of the nonionic surfactant

Triton X-100. The foreign ions such as Fe (Ⅲ)

Al (Ⅲ)

Ca (Ⅱ)

etc. produced interference on colour reaction but Al (Ⅲ) and Ca (Ⅱ) could be masked with masking reagents. The outstanding advantages of this method are simplicity

high sensitivity and good selectivity as well as the complex is very stable. The reagent is suitable for applied to the direct spectrophotometric determination of microgram amounts of U (Ⅵ) and the results are found to be satisfactory.