An outwardly propagating spherical flame was used to characterise the influence of water loading on the premixed combustion of an applied high CO/H-2 ratio syngas fuel blend (converter gas). A nonlinear extrapolative technique was used to obtain values of laminar flame speed for combustion with air, for varying temperature, pressure and equivalence ratio. With increased attention given to the accurate measurement of laminar flame speed, a concerted effort was made to quantify experimental uncertainty, and a detailed methodology is presented. Change in relative humidity was shown to have a substantial impact on laminar flame speed for the syngas, increasing measured values by up to 70% from the driest cases. This observed increase results from the dissociative influence of H2O addition, and enhancement in the formation of chain carriers that catalyse CO oxidation, increasing net heat release rate. In addition to relative humidity, the decoupled influences of initial temperature and pressure were investigated parametrically; holding the mass ratios of fuel and H2O constant for a step change in condition. Temperature rise was shown to enhance H2O induced acceleration, with greater relative change in heat release rate for a corresponding drop in flame temperature, and the opposite effect observed for increased pressure. The effect of water addition was shown to be non-monotonic, with flame speed reduction achieved at the highest water loadings for the hottest tests, and discussed as a function of initial CO/H-2 ratio. Attention was given to the dominant reaction kinetics, with the performance of several published reaction mechanisms evaluated against experimental data using CHEMKIN-PRO; with flame speed consistently overpredicted when H2O was added to the mixture. A modified reaction mechanism is presented for the humidified combustion of high CO/H-2 mixtures, changing the rate parameters of two chain branching reactions to give higher relative indeterminate H2O formation, and a reduction in OH carriers. Results obtained using the modified mechanism demonstrate improved agreement with all experimental data presented here and from a previous study, including changes in H2O concentration at elevated temperatures and pressures. The results also highlight relative humidity as a potential source of error in the experimental measurement of u(L), significant for fuels comprising large CO fractions, but also potentially for other gaseous fuels, emphasising that relative humidity should be carefully considered when comparing experimental data. (C) 2015 The Authors. Published by Elsevier Inc. on behalf of The Combustion Institute. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).