Processes with metallic catalysts depend on the metal composition and the specific surface area of metallic materials. The formation of colloidal solutions containing metal nanoparticles at different ratios of silver and gold in microreactors was studied in order to check the possibility of preparing such nanoparticles under micro-flow-through conditions. Therefore, tetrachloroauric acid (in concentrations between 1 and 5 mM) and silver nitrate (in concentrations between 5 and 50 mu M) were reduced with sodium borohydride or ascorbic acid. Additions of a water soluble polymer (polyvinyl alcohol) and sodium hydrogen carbonate influence the reaction rates. Static micromixers (made by microlithographic Si/glass chip technology) and PTFE tubes were applied for the flow-through experiments in the flow rate range between 5 mu l/min and 4 ml/min. It was shown that the optical properties of the colloidal product solutions depend both on the mixing order of the reactant solutions and on the over-all flow rates. The catalytic activity of the obtained nanoparticles and nanostructured precipitates was evaluated by bleaching of organic dyes with potassium peroxodisulphate in the presence of the nanoparticle material after centrifugation, rinsing and redispergation. Bleaching rates were measured by cycling spectrophotometry. Significant enhancements of bleaching rate were found in case of silver, palladium and iron-containing catalyst nanoparticles. (C) 2007 Elsevier Ltd. All rights reserved.