Abstract: A fading spectrophotometric method for the determination of trace mercury has been developed. In 0.01 mol/L H₂SO₄ medium, Hg²⁺ reacts with excess I^- and safranine T(ST) to form a stable ion-association complex ( ₂ ), which leads to the fading of ST. The maximum fading wavelength locates at 519 nm. The fading degree of the system is linear with the concentration of Hg²⁺ in the range of 0~1.35 μg/mL. Apparent molar absorption coefficient is 1.24×10⁵ L/(mol·cm) with detection limit of 2.26 μg/L for mercury. Recoveries of Hg²⁺ are 98.3%~102.6% with precision of 1.2%~1.7% RSD (n=5). The method has been applied in the determination of trace mercury in river water samples after enrichment and separation with thiol cotton fiber.