Surface properties of the silica/NaClO4 interface were studied by potentiometric titration, Na adsorption versus pH, electrokinetic potential measurements, and adsorption of cadmium using the adsorption edge technique. The silica/NaClO4 interface is characterized by low negative charge, without clear intersection point on titration and electrokinetic potential curves at various ionic strengths, which tend towards zero charge for pH <4. Cadmium ions adsorption occurs through competition with sodium ions for the dissociated sites of silica. At the highest studied electrolyte concentration (10(-1) M) adsorption edges are dominated by precipitation of hydrolyzed cadmium. An effect of silica surface heterogeneity on cadmium adsorption was detected for low electrolyte concentration (10(-3) M). The data collected for this later electrolyte concentration are used for thermodynamic modeling of the cadmium adsorption at the silica/NaClO4 interface, performed in the second part of this paper. (C) 2002 Elsevier Science B.V. All rights reserved.