Abstract: It was found that in an ammoniacal buffer of pH 10.4 and in the presence of non-ionic surfactant Triton X-100,the new chromogenic reagent o-methylbenzenediazoaminoazobenzene (abbreviated as o-MDAAB) reacted with Tl(Ⅲ) to form a reddish colored complex,with its absorption maxima at the wavelength of 516 nm;and the mole ratio of Tl(Ⅲ) to o-MDAAB in the complex was found to be (Tl∶R) 1∶2.Beer′s law was obeyed in the concentration range of 0 to 20 μg of Tl(Ⅲ) per 25 mL of solution.The apparent molar absorptivity was found to be 1.69×105L·mol-1·cm-1.Selectivity of this method was improved by the addition of trisodium citrate.The proposed method has been applied to the determination of thallium in tap water and simulated water samples,giving RSD′s of 1.05% and 0.29%,and recoveries of 97.5% and 103.7% respectively.