The afterglow of a microwave plasma (2.45 GHz) of dihydrogen is used for the preparation of zeolite-supported gold-based metallic catalysts. It contains hydrogen atoms at a sufficiently low temperature for the formation of nanoparticles, whereas the conventional reduction of gold at high temperature often leads to large particles. The study concerns the search for optimal conditions for the preparation of monometallic (Au or Pt) and bimetallic (Pt-Au) catalysts. The following experimental conditions are examined: position of the sample in the afterglow, exposure time, gas pressure, composition of the sample for bimetallics, type of zeolite (NaY or HY). Direct images of supported metal particles are obtained by transmission electron microscopy (TEM). The sizes and dispersion of the particles are determined from TEM photographs (mean diameter d(m), volumic diameter d(v) and dispersion D). The dispersion of platinum atoms is also determined by dihydrogen chemisorption. The particles obtained are less than 5 nm in size, and are stable to thermal treatment. (C) 2003 Elsevier B.V. All rights reserved.