Xylenol orange is proposed as new sensitive reagent for the colorimetric determination of selenium and tellurium. The reagent forms a pink colored radical cation with selenite and tellurite instantaneously at room temperature in slightly acidic medium. The pink species exhibits an absorption maximum at 568 and 569 nm with a molar absorption coefficient of 2.69 × 104and 2.82 × 1041 mol−1cm−1and Sandell sensitivity of 2.94 and 4.52 ng cm−2for selenite and tellurite, respectively. A 10-fold molar excess of the reagent is necessary for the development of the maximum color intensity. Beer's law is obeyed over the concentration range 0.10–5.50 and 0.13–6.38 μg ml−1of selenium and tellurium, respectively with an optimum concentration range of 0.35–5.10 and 0.40–6.00 μg ml−1, respectively using Ringbom method. The effects of acidity, time, temperature, order of addition of reagent, reagent concentration and the tolerance limit of the method towards various cations and anions usually associated with selenium and tellurium are reported. The proposed method is extremely sensitive, reproducible and has been satisfactorily applied to the determination of trace amount of selenium in polluted water, plant material and medicated shampoo.