Chemical kinetic mechanisms for simulation of syngas (H-2 + CO) combustion are important for development of efficient practical applications, such as gas turbines. A useful syngas mechanism has to be able to accurately predict laminar burning velocities, ignition delays, and oxidation of gas mixtures of varying composition over a range of temperatures and pressures. In the present work, the performance of a new H-2/CO combustion mechanism is analyzed. The mechanism is built on reaction rate constants chosen from the most accurate available kinetic data, and this is thoroughly discussed. The mechanism is validated for a wide range of experimental data from the literature. Particular attention is paid to chemistry of the species HOCO, produced from CO + OH reaction and removed by decomposition or radical reactions. Most available syngas mechanisms do not include HOCO because it is only expected to be of importance at some extreme high-pressure and low-temperature conditions. The species is, however, essential in hierarchically extended mechanisms for small oxygenated hydrocarbons, and its influence on the H-2/CO subset of reactions needs to be further understood to ensure accurate mechanism development for a range of fuels. In the present study, it is found that inclusion of the HOCO reaction subset does not alter the model predictions of laminar burning velocities, ignition delay times, or oxidation. Sensitivity analysis reveals that HOCO production, its thermal decomposition, and reaction with O-2 are among the 20 most sensitive reactions for conditions of low temperatures and high CO concentrations but with insignificant magnitude of sensitivity compared to that of the major sensitive reactions.