An experimental and numerical study on thermal and chemical effects of water vapor addition on the laminar burning velocities of syngas was conducted using a constant-volume chamber and CHEMKIN package. The experimental conditions in the present study for CO/H-2/air/H2O mixtures with hydrogen fraction in syngas were from 5% to 50%, initial temperature of 373 K, pressures of 0.1 and 0.5 MPa and water dilution ratios from 0% to 30%. The measured laminar burning velocity data were compared with simulations with three mechanisms, the San Diego mechanism and those of Davis et al. and Li et al. The experimental data showed a reasonable agreement with the calculated values at low and high pressures when the content of H-2 in the fuel was low. However, when H-2/CO ratio in the fuel was higher (75/25 and 95/05), all three mechanisms overpredicated the laminar burning velocity for fuel-rich mixtures. Sensitivity analysis was performed to identify the possible sources of discrepancy between the experimental data and calculated results. Furthermore, chemical effects of H2O on the laminar burning velocities at various CO/H-2 ratios when water was added into the mixtures were studied. For syngas with CO/H-2 ratio of 50/50, water had a weak inhibiting effect on the chemical reaction of the mixtures. For the higher CO/H-2 ratios, water addition accelerated the chemical reaction and this positive effect became more significant for the syngas with the increased CO/H-2 ratio. Different trends were explained using the consumption path analysis.