Using a combination of mass spectrometry, in situ quick extended X-ray absorption fine structure, high-resolution transmission electron microscopy, and hydrogen chemisorption, we studied the reactions taking place during different pretreatments of the catalyst precursor [pt(2+)(NH3)(4)](NO3-)(2) impregnated on high-surface-area SiO2 (400 m(3)/g). Direct reduction in hydrogen leads to the formation of Pt metal particles in the temperature range of 150-200degreesC in a fast process. The reduction is accompanied by sintering of the platinum particles, leading to relatively large particles, with an average particle size of approximately 14-16 Angstrom. Autoreduction in helium leads to multiple steps in the reduction. Around 210 and 240degreesC, NOx released due to the decomposition of NH4NO3, formed during heating up to 180-200 degreesC, reduces the catalyst precursor at a high rate. At higher temperatures, the reduction continues slowly through an autoreduction of the Pt(NH3)(2+)(x) complex. The slow reduction rate suggests a nonmobile species. Accordingly, the final metal-particle size is small, with particles of 10-12 Angstrom. Calcination-reduction results in large particles via a similar decomposition of NH4NO3. Particle-size distribution after autoreduction is considerably smaller than after direct reduction. The key to obtaining small particles with a relatively narrowsized distribution is to avoid the formation of mobile species. With impregnated [pt(2+)(NH3)(4)](NO3)(2), this is best achieved by autoreduction.