A spiral microreactor is analyzed for combustion of hydrogen on Pt catalyst using 3D computational fluid dynamics (CFD) simulations with detailed mechanisms for both catalytic and gas phase reactions. A spiral microreactor comprises of a single channel that is curled-up in a spiral, starting at the center and spiraling outwards. A comparison of the spiral with a conventional straight-channel reactor is presented. Maximum temperature attained in the spiral microreactor, investigated for equivalence ratios within the range of 0.3 to 0.65 under non-adiabatic conditions (with heat losses by convection and radiation), is found to be greater than the adiabatic flame temperatures. Comparison with a straight-channel reactor of equivalent dimensions shows higher temperature and higher contribution of homogeneous chemistry in the spiral microreactor. The nature of homogeneous reactions in the presence of catalytic micro reactor is presented. Combustion characteristics presented herein indicate that the preheating of reactants and protection of reaction zone in the spiral reactor make it superior to the straight channel reactor. The effect of inlet velocity is presented and the effect of heat recirculation on homo-catalytic interactions is analyzed. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.