A series of well-defined catalysts based on platinum nanoparticles supported on amorphous silica-alumina with varying composition was prepared by incipient wetness impregnation. Quantitative structural characterization of alumina and aluminosilicate domains by Al-27 (3Q) MAS NMR spectroscopy showed that the concentration of Bronsted acid sites determined by pyridine adsorption is related solely to the concentration of the aluminosilicate domain. However, only a very small fraction of the tetrahedrally coordinated aluminum led to the formation of Bronsted acid sites, because the negative charge on most of the tetrahedral Al-O was balanced by cationic aluminum oxide clusters. The preparation method led to small, uniform (0.6-0.8 nm) Pt particles on all aluminum-containing supports. (C) 2007 Published by Elsevier Inc.