The adsorption of Pt(NH3)(4)(+2) onto five different amorphous silicas (fumed and precipitated) was studied as a function of pH and concentration of platinum in solution. The ability of silica to protonate and deprotonate as a function of pH leads to an electrostatic attraction of the +2 platinum cation in the basic pH range, where the silica surface is negatively charged. Adjusting for the differences in surface area of the various silicas, and since all have about the same point of zero charge (PZC), the adsorption behavior is similar for all materials. Platinum uptake as functions of pH and metal concentration is reasonably simulated by the revised physical adsorption (RPA) model using the same set of independently measured parameters for all five silicas. This is the same model that has been previously employed to describe the adsorption of anionic Pt chloride complexes over alumina in the acidic pH range [Chem. Eng. Sci. 56 (2002) 3491]. The proton balance included in the model also accounts for shifts in pH, which are virtually the same with and without metal in solution. Because of this, and because the maximum extent of Pt adsorption appears to have a steric limit and is much less than that predicted by an "ion-exchange" mechanism, we believe that "electrostatic adsorption" is a more precise description of the adsorption mechanism. In a second paper among other issues we show that platinum complexes adsorbed onto silica at the optimal pH of about 10 remain 100% dispersed after reduction. (C) 2004 Elsevier Inc. All rights reserved.