A new, reliable, surfactant-sensitized method is described for the spectrophotometric determination of total and dissolved inorganic mercury at trace levels. The method is based on the ternary complex formation of Hg(II) with 2-(2-thiazolylazo)-p-cresol and a cationic surfactant, cetylpyridinium chloride, at pH 9.5. The detection and quantification limits of the method with surfactant were 6 and 19 mu g L-1, while those of the method without surfactant were 22 and 67 mu g L-1 respectively. The method obeys Beer's law at up to approximately 10 mg L-1 Hg(II) in an aqueous surfactant medium. The relative errors and relative standard deviations of the method were 2.2% and 3.2%, while those of the direct method were 11% and 4.4% (1.0 mg L-1, n = 10), respectively. The accuracy and reliability were examined by the recoveries of spiked solutions at 3 and 5 mg L-1 for the determination of mercury in tap water, drinking water, dental unit wastewater, and artificially prepared model water samples. It was found that the results were very good and comparable for both the present method and the modified dithizone method, which was used as an independent reference method. Speciation studies for binary mixtures containing Hg(I) plus Hg(II) ions at known concentration ratios were also conducted.