The preparation and characteristics of thiol-modified silica and its application to the preconcentration and determination of trace amounts of copper and cadmium by flame atomic absorption spectrometry are described. For the preparation of modified silica, the untreated silica suspended in methanol was impregnated with 3-(trimethoxysilyl)-1-propanethiol to obtain thiol-modified silica and the methanol was evaporated under vacuum. The residual silica was dried at 150-degrees-C and washed with distilled water until the washings appeared clear. The surface area of thiol-modified silica was 410 m2/g, whereas the copper capacity was about 0.022 mmolCu/g silica. Batch and column methods were used for the separation and concentration of the above-mentioned metals. These metals were quantitatively retained on the adsorbent at acidic media. In the batch method, the effects of pH, shaking time and the kind of buffer on adsorption of copper and cadmium were investigated. The shaking time of 30 min was sufficient for a quantitative adsorption of the metals. In the column and batch procedures, both copper and cadmium adsorbed on the modified silica were quantitatively recovered with 2 M hydrochloric acid. Sodium chloride (up to 1%) had no effects on the recoveries.