Ignition delays were measured in a shock tube for syngas mixtures with argon as diluent at equivalence ratios of 0.3, 1.0 and 1.5, pressures of 0.2, 1.0 and 2.0 MPa and temperatures from 870 to 1350 K. Results show that the influences of equivalence ratio on the ignition of syngas mixtures exhibit different tendency at different temperatures and pressures. At low pressure, the ignition delay increases with an increase in equivalence ratio at tested temperature. At high pressures, however, an opposite behavior is presented, that is, increasing equivalence ratio inhibits the ignition at high temperature and vice versa at intermediate temperature. The affecting degree of equivalence ratio on ignition delay is different for each mixture at given condition, especially for the syngas with high CO concentration. Sensitivity analyses demonstrate that reaction H + O-2 = O + OH (R1) dominates the syngas oxidation under all conditions. With the increase of CO mole fraction, reactions CO + OH = CO2 + H (R27) and CO + HO2 = CO2 + OH (R29) become more important in the syngas ignition kinetics. With the increase of pressure, the reactions related to HO2 and H2O2 play the dominate role. The opposite influence of equivalence ratio on ignition delay at high- and intermediate-temperatures is chemically interpreted through kinetic analyses. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.