The experimental evaluation of an autothermal microchannel reactor for H-2 production from NH3 decomposition is described. The reactor design incorporates an autothermal approach, with added NH3 oxidation, for coupled heat supply to the endothermic decomposition reaction. An alternating catalytic plate arrangement is used to accomplish this thermal coupling in a cocurrent flow strategy. Detailed analysis of the transient operating regime associated with reactor start-up and steady-state results is presented. The effects of operating parameters on reactor performance are investigated, specifically, the NH3 decomposition flow rate, NH3 oxidation flow rate, and fuel-oxygen equivalence ratio. Overall, the reactor exhibits rapid response time during start-up; within 60 min, H-2 production is approximately 95% of steady-state values. The recommended operating point for steady-state H-2 production corresponds to an NH3 decomposition flow rate of 6 NL min(-1), NH3 oxidation flow rate of 4 NL min(-1), and fuel-oxygen equivalence ratio of 1.4. Under these flows, NH3 conversion of 99.8% and H-2 equivalent fuel cell power output of 0.71 kW(e), is achieved. The reactor shows good heat utilization with a thermal efficiency of 75.9%. An efficient autothermal reactor design is therefore demonstrated, which may be upscaled to a multi-kW H-2 production system for commercial implementation.