The laminar flame speeds of premixed CO/H-2/O-2/H2O were measured using a Bunsen burner method. The effects of the equivalence ratio and CO/H-2 ratio on the laminar flame speeds were analyzed under various conditions with the initial temperature of 400 K, H2O content in the mixture of 47%. A modified CO/H-2 combustion reaction mechanism in O-2/H2O atmosphere was proposed based on recently published kinetic and thermodynamic data. The laminar flame speed was calculated using the Davis, Li, and modified mechanisms. At higher H-2 content, the laminar flame speeds simulated with the Davis and Li mechanisms were lower than the experiment data, but the modified mechanism results agreed well with the experimental ones. A sensitivity analysis of the CO/H-2 combustion mechanism was performed in O-2/H2O atmosphere and air atmosphere to identify the critical elementary reactions. The chemical effects of H2O on CO/H-2/O-2 combustion were analyzed using integrated species consumption path analysis. For CO content >42% conditions, H2O addition accelerated reaction O + H2O = OH + OH and promoted combustion, for CO < 42% conditions, it inhibited combustion by reducing the reaction rate of O + H-2 = H + OH. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.