The mechanism of formation of Pt@SiO2 as a model of core–shell nanoparticles via water-in-oil reverse microemulsions was studied in detail. By controlling the time of growth of Pt precursors, Pt(OH)x, after hydrolysis in NH3 aq. before adding SiO2 precursor (TEOS), Pt nanoparticles with a narrow size distribution were produced, from ultrafine metal nanoparticles (<1 nm) to 6 nm nanocrystals. Separately, the thickness of SiO2 was controllably synthesized from 1 to 15 nm to yield different Pt@SiO2 materials. The Pt@SiO2 core–shell catalysts exhibited a higher rate of CO oxidation by one order of magnitude with a positive order regarding CO pressure. The SiO2 shell did not perturb the Pt chemical nature, but it provided different coverage of CO in steady-state CO oxidation. © 2016 Elsevier Inc.