A new method for the direct determination of trace mercury in aqueous solution with high selectivity and sensitivity is reported. In the presence of Triton X-100 and NH3.NH4Cl medium, a new chromogenic reagent p-azobenzenediazo- aminoazobenzene sulfonic acid (ADAAS) forms a red complex with mercury(II). The molar absorptivity is 1.76×105l. mol−1cm−1at 528 nm. Beer's law is obeyed in the range of 6.0×10−8- 6.0×10−6mol/L for Hg(II). Only cadmium interferes in the determination of mercury, and it can be eliminated by masking with nitrilotriacetic acid. The proposed method has been successfully used for the determination of trace amounts of mercury in waste water.