The temperature-programmed decomposition of platinum tetrammine hydroxide mixed with or exchanged on silica in a flow of neutral, oxidizing, or reducing gas is studied using mass-spectrometric analysis of the effluent. A unified interpretation mechanism is proposed whatever the nature of the gas. It involves (equivalent toSiO)(2)Pt(NH3)y with y approximate to 2 as the intermediate species. This species anchors the particles defined at the drying step. Anchoring takes place at 100-150 degreesC whatever the gas. The subsequent decomposition of this species leads either to Pt-(0) (reducing gas), mobile PtO (oxidizing gas), or a mixture of both (neutral gas). The dispersion of the platinum particles ranges between 70 and 80% for metal loading up to 5% when the decomposition is performed under the reducing gas up to 500 degreesC. In the neutral gas, the dispersion ranges from 42 to 72% and depends on the final temperature and on metal loading. In an oxidizing gas, the dispersion can be about 65% provided that the final temperature does not exceed 250 degreesC. Subsequent reduction at a higher temperature reproduces the decomposition under the reducing gas. When the final temperature is raised above 250 degreesC in the oxidizing gas, the dispersion decreases and falls to 20% at 500 degreesC. It is proved that PtO is responsible for low dispersions. Conversely, under reducing conditions, the size of the particles obtained upon drying is kept unchanged. The proposed mechanism explains why a neutral gas gives intermediate results that depend on pressure and metal loading. In the case of high metal loading, decomposition under the reducing gas leads too small particles grouped together, although dispersion remains within 70-80%. (C) 2003 Elsevier Inc. All rights reserved.