A novel “sandwich” microreactor designed as a thin (∼ 300 $\mu$m) porous plate of sintered metal fibers (SMF), sandwiched between metallic plates is reported. The SMF surface was coated by Fe-ZSM-5 thin film (< 2 $\mu$m) rendering a catalyst highly active in the decomposition of N2O. The 3D open microstructure of SMF plates presents a low pressure drop during the passage of reacting gases. The high thermoconductivity of metallic SMF improves the heat transfer, avoiding hotspot formation during exothermic reactions. The temperature measured in the middle and at the outlet of the reactor confirmed the isothermal reactor operation. The sandwich microreactor showed high permeability and a narrow residence time distribution close to an ideal plug-flow reactor. The kinetics of the N2O decomposition was studied and the reaction was shown not to be limited by mass transfer when conducted in the microreactor.