A genetic algorithm (GA) was proposed and validated for the optimal extraction of a sub-mechanism for H2/CO/CH4/CO2 mixtures from the detailed Aramco1.3 chemical kinetics mechanism (Metcalfe et al., 2013), which was developed for C1–C5 hydrocarbons and oxygenated fuels. Ninety ignition delay time data involving mixtures containing H2, CO, CH4, CO2, N2, and H2O at wide range of experimental conditions were chosen as optimization targets to guide the GA, so that the final reduced mechanism was able to fully describe the combustion characteristics of syngas and biogas fuel mixtures. The final reduced mechanism for H2/CO/CH4/CO2 fuel mixtures comprised of 72 species and 290 reactions (reduced from 325 species and 2067 reactions), and it was extensively validated against experimental results, such as measured ignition delay times and laminar flame speeds, over a wide range of operating conditions. The excellent agreement between the reduced mechanism and Aramco1.3 mechanism in predicting the combustion properties of H2/CO/CH4/CO2 mixtures with maximum relative error values of only 0.9% and 2.75%, respectively for the ignition delay time and laminar flame speed results, indicates that the proposed reduced mechanism can be used for predicting the combustion characteristics of biogas and syngas fuel mixtures. Furthermore, it was observed that the reduced mechanism shows excellent agreement with the Aramco1.3 mechanism in predicting the ignition delay time of mixtures with added ethane and propane. Therefore, the proposed reduced mechanism represents the most up-to-date detailed chemical kinetics mechanism for biogas and syngas fuel mixtures, and it can also be used for predicting the combustion properties of natural gas with impurities such as ethane and propane. The reduced mechanism agreed so well with the Aramco1.3 mechanism in predicting the combustion properties of H2/CO/CH4/CO2 mixtures, where both mechanisms performed identically in over predicting the ignition delay time for H2/CO/CO2, CO2, H2/CO/CH4, and H2/CO/CH4/CO2/H2O mixtures at several experimental conditions. These observations were also reported by Lee et al. (2015), where they proposed two new rate constants for H + O2(+CO2) = HO2(+CO2) and CH4 + OH = CH3 + H2O to reconcile the discrepancies observed. These two new rate constants were assessed in this study by incorporating the modified rate constants into the 290Rxn mechanism (referred as 290Rxn-V1), where it was found that the modified rate constants did improve the ignition delay time predictions. However, the two proposed rate constants did not improve the predictions of the laminar flame speed for mixtures with a high CO2 content at high equivalence ratios (ϕ > 1.2). Therefore, optimization of the rate constants in the 290Rxn mechanism is highly recommended to further improve its agreement with experimental data for biogas and syngas fuel mixtures.