To determine the auto-ignition and combustion mechanisms and the components of syngas that are applicable to homogeneous charge compression ignition (HCCI) engines, the combustion characteristics and the chemical reaction process in an HCCI engine were studied numerically and experimentally using mock syngas with various mixtures of the fuel components. The mock syngas consisted of hydrogen (H2) and carbon monoxide (CO) as the main combustible components, nitrogen (N2) and carbon dioxide (CO2) as incombustible components and a small amount of methane (CH4), assuming the composition of the gas was produced from wood by thermochemical conversion processes. The oxidation reaction process was analyzed numerically using CHEMKIN-PRO. Further experiments were conducted to investigate the validity of the calculated results. Primarily, the effects of hydrogen and carbon monoxide on auto-ignition and combustion were investigated. Auto-ignition timing mainly depends on the in-cylinder gas temperature and the auto-ignition temperature is approximately 1100 K, which is the same as that of hydrocarbon fuels. It has been shown that the rate of combustion following auto-ignition is mainly determined by the hydrogen and carbon dioxide contents. The time interval from 10% to 90% conversion is longer for hydrogen than for carbon monoxide. The amount of hydrogen begins to decrease before that of carbon monoxide. The amount of carbon monoxide, however, decreases rapidly. The combustion duration can be roughly estimated from the ratio of H2 to CO2 in the fuel.