Homogenous charge compression ignition (HCCI) combustion has the potential to work with high thermal efficiency, low fuel consumption, and extremely low NOx-PM emissions. In this study, zero-dimensional single-zone and quasi-dimensional multi-zone detailed chemical kinetics models were developed to predict and control an HCCI combustion engine fueled with a natural gas and reformer gas (RG) blend. The model was validated through experiments performed with a modified single-cylinder CFR engine. Both models were able to acceptably predict combustion initiation. The result shows that the chemical and thermodynamic effects of RG blending advance the start of combustion (SOC), whereas dilution retards SOC. In addition, the chemical effect was stronger than the dilution effect, which was in turn stronger than the thermal effect. Furthermore, it was found that the strength of the chemical effect was mainly dependent on H-2 content in RG. Moreover, the amount of RG and concentration of species (CO-H-2) were varied across a wide range of values to investigate their effects on the combustion behavior in an HCCI engine. It was found that the H-2 concentration in RG has a more significant effect on SOC at lower RG percentages in comparison with the CO concentration. However, in higher RG percentages, the CO mass concentration becomes more effective than H-2 in altering SOC. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.