Platinum nanoparticle, of interest because of their high catalytic activity, were prepared in an aqueous system using high density ultrasound (200 kHz and 6 W cm ²). The nanoparticles were found by researchers from Osaka Prefecture University (Y. Mizukoshi, R. Oshima, Y. Maeda and Y. Nagata, Langmuir, 1999, 15, (8), 2733–2737) to be smaller and more regular than those made radiochemically. Stable, homogeneously spherical, monodispersed Pt nanoparticles, of average diameter 2.6 nm, were formed at the rate of 26.7 μM min¹ in a Pt(II)-sodium dodecyl sulfate system. Three kinds of reducing species are proposed to be formed in the sonicated system, near and/or in the hot bubbles, and these then react with the PtCl₄² complexes to form the platinum nanoparticles.