Adsorption behavior of platinum (II) ammine complex ions onto silica gel was investigated at 30 degrees C in basic ammoniacal solution with varying concentrations of ammonium chloride. Two reactions were sugested for the platinum adsorption as follows: GNH(4) + Pt(NH3)(4)(++) + Cl- reversible arrow GPt(NH3)(4)Cl + NH4+ K-1 GNH(4) + GH + Pt(NH3)(4)(++) + NH3 reversible arrow G(2)Pt(NH3)(4) + 2 NH4+ K-2 where GH denotes silanol groups on silica gel. The composite process of these reactions gives the following equation, and it agreed with experimental data. [GPt]/[Pt++]=K-1[GNH(4)][Cl-]/[NH4+]+K-2＇[GNH(4)](2)/[NH4+](2) The bidentate adsorption of platinum ammine complex ion onto silica gel retains a site of ammonia and an adjacent site of free silanol group. In the reduction process of platinum ammine complex ions adsorbed on silica gel into platinum metal, calcination in the air at 350 degrees C prior to reduction in H-2 causes an exothermic reaction due to the combustion of ammonia adsorbed. The exothermic reactin induces sintering of platinum particles. The direct reduction in H-2 at 350 degrees C led to highly dispersed platinum on silica gel.