A new simple, very sensitive, selective and accurate procedure for the determination of trace amounts of thallium(III) by solid-phase spectrophotometry (SPS) has been developed. The procedure is based on fixation of Tl(III) as quinalizarin ion associate on a styrene-divinylbenzene anion-exchange resin. The absorbance of resin sorbed Tl(III) ion associate is measured directly at 636 and 830nm. Thallium(I) was determined by difference measurements after oxidation of Tl(I) to Tl(III) with bromine. Calibration is linear over the range 0.5-12.0μgL(-1) of Tl(III) with relative standard deviation (RSD) of 1.40% (n=10). The detection and quantification limits are 150 and 495ngL(-1) using 0.6g of the exchanger. The molar absorptivity and Sandell sensitivity are also calculated and found to be 1.31×10(7)Lmol(-1)cm(-1) and 0.00156ngcm(-2), respectively. The proposed procedure has been successfully applied to determine thallium in water, urine and serum samples.