An experimental investigation was carried out to assess the effect on engine operation of the addition of 20% hydrogen by volume to natural gas. Blends of hydrogen and natural gas are referred to as hythane in this report. Three groups of testing were conducted. All the tests were conducted at light loads similar to those in urban driving. The first group of tests were conducted using a 2.0 liter Nissan four cylinder engine to measure the increase in flame front propagation rate when fueled with hythane as compared to pure methane (simulating natural gas). The second group of tests were conducted with a 1.6 liter Toyota four cylinder engine to measure the changes in emissions and thermal efficiency comparing hythane operation to pure methane operation. The third group of tests were conducted with a 1.6 liter Toyota four cylinder engine operating on pure methane to measure changes in emissions and thermal efficiency with and without a pre-chamber installed in the normally hemispherical combustion chamber. The second and third group of tests were conducted to ascertain the relative benefits of modifying the fuel as opposed to modifying the engine configuration.