In this paper critical steps in the fabrication process of a microreactor for high-temperature catalytic partial oxidation gas phase reactions are evaluated. The microreactor contains a flow channel etched in silicon, capped with an ultrathin composite membrane consisting of silicon and silicon nitride layers, on which on the topside thin-film heaters and sensors, and on the other side a thin-film catalyst patch are placed. The membrane is designed to have specific heat conductivity and mechanical properties. The paper focuses on three fabrication issues: definition and etching of sub-micron uniform single-crystalline silicon membranes, deposition of well-defined heater structures and temperature sensors on a thin composite membrane, and deposition of well-defined catalytic patches on the same membrane. For the latter two processes novel micromachined shadow masks were developed. Preliminary experiments on the controlled oxidation of hydrogen gas in the explosive regime are discussed, which experiments confirm that heat management in the microreactor is excellent.