Silica gels modified with 3-aminopropyltriethoxysilane or 3-mercaptopropyltrimethoxysilane groups have been developed for the preconcentration of copper and cadmium prior to their determination by flame atomic absorption spectrometry. The surface areas of the modified silica gels were determined by the Brunauer-Emmett-Teller (BET) method to be 290 m2 g-1 of the amino-modified silica and 410 m2 g-1 of the thiol-modified silica. Batch and column methods were used for the separation and concentration of copper and cadmium. These metals were quantitatively retained on both the modified gels in slightly acidic media. In the batch method, the effects of pH, shaking time and type of buffer on the adsorption of copper and cadmium were investigated. In the batch and column procedures both copper and cadmium adsorbed on the silicas were quantitatively recovered (relative standard deviation of 2-6%) using 2 mol dm-3 hydrochloric acid even in the presence of sodium chloride up to a concentration of 1.0%.