Macroporous silicon with millions of parallel rnicrochannels per square centimeter can be used as a micromonolithic support for high-speed catalysis in miniaturized systems, opening a new and exciting field of research and application. We used macroporous silicon membranes with >10(6) parallel microchannels with a diameter of 3.3 mu m and depth/width ratio >60 as microreactors for the generation of hydrogen through the steam reforming of ethanol. The microchannels walls were coated with a thin layer of Co3O4-ZnO catalyst by a complexation-decomposition method, resulting in a specific inner surface of 10(5)-10(6) m(2) m(-3). A constant flow of 20 mu L s(-1) H-2 (0.054 mmol min(-1) H-2) was obtained from a liquid water-ethanol mixture at 773 K with a steam-to-carbon ratio of S/C = 3 and LHSV = 2 x 10(4) h(-1) (0.1 mL min(-1)). The turnover frequency under these conditions was ca. 1.1 s(-1), based on surface Co atoms. (C) 2008 Elsevier Inc. All rights reserved.