The performance of a four-stroke Honda GC160E spark ignition (SI) internal combustion (IC) engine operating on landfill gas (LFG) was investigated, as well as the impact of H-2 and CO (syngas) addition on emissions and engine efficiency. Tests were performed for engine loads from 0.2 to 0.8 kW over a range of CO2, to CH4 ratios (0-0.50). In addition, variation across both the syngas content (up to 15%) and the ratio of H-2 to CO in the syngas (H-2/CO = 0.5, 1, and 2) were tested. Catalytic testing provided reactor data on the amount of syngas and H-2/CO ratios that can be obtained by autothermally reforming LFG. The emissions obtained from the test engine fueled with the simulated LFG were found to be comparable to emissions from commercial LFG to energy (LFGTE) systems currently deployed. Syngas addition was found to not only significantly reduce CO, unburned hydrocarbon (UHC), and NOx emissions but also improve brake efficiency of the engine. CO emissions were reduced from 802 to 214 ppm for a 5% syngas addition and to 230 and 247 ppm for 10 and 15% syngas addition, respectively. UHC emissions were reduced from 113 ppm to approximately 12 ppm for all amounts of syngas addition. Syngas addition decreased NOx from 100 to 62 ppm for 5% syngas and 71 and 76 ppm for 10 and 15% syngas, respectively. Finally, the brake efficiency increased by approximately 10% with the addition of 5% syngas.