The design and fabrication of a gas-phase microreactor that is based upon a multilayer laminate microelectronics structure is described. The reactor is a key component in an integrated system whose platform utilizes a commercial computer chassis with modular boards to perform the required process functions. The design combines knowledge from earlier laminate microstructures with new prototyping concepts for incorporation of various on-board devices. A 3-D finite element simulation model was used to identify various design improvements. The final device contains two parallel reaction channels on a chiplike die in which a 1 mu m platinum film catalyst is deposited on the underside of a silicon nitride membrane. Seven platinum heaters and temperature sensors are evenly distributed along the top side of the silicon nitride membrane. Electrical contacts for the on-board control and sensing devices are achieved through various pins that are distributed around the reactor die. The experience and knowledge gained in developing the final reactor device is utilized in Part 2 of this series for reactor packaging and development of the integrated system modules.