The activity and the heat transfer characteristics of several microstructured reactors have been compared in the ammonia oxidation on Pt catalyst. The main parameters which influence reactor performance are catalyst loading, temperature, and the intrinsic conductivity of the reactor material, In case of aluminum as a reactor material, hot spot temperatures were within 5 degreesC at full conversion of 6 vol.% NH3. Temperature gradients were considerably larger when the microreactor was made from pure platinum due to the smaller intrinsic material conductivity. As a result, the maximum N2O selectivity was by 20% lower than in the case of the aluminum-based reactor due to considerable differences in the selectivities between the central and wall channels. Experimental data obtained on the above microreactors were used to design an externally cooled cross flow microreactor/heat-exchanger operating at almost isothermal conditions even with a reaction mixture corresponding to an adiabatic temperature rise of about 1400 degreesC. Such system can provide new opportunities for improvement of existing gas/solid catalytic processes with strongly exothermic reactions.