An experiment was conducted to evaluate the efficiency and emissions of an engine fuelled with a mixture of natural gas and approximately 15% hydrogen by volume. This mixture, called Hythane™, was compared with natural gas fuel using engine efficiency and engine-out emissions at various engine operating conditions as the basis of comparison. Throughout most of the experiment, fuel mixtures were slightly rich of stoichiometry. It was found that at low engine loads, using the same spark timing, engine efficiency increased under HythaneTM fuelling but at higher engine loads, natural gas and Hythane™ had the same efficiency. At low engine speed and load conditions with the same spark timing, engine-out total hydrocarbon (THC) emissions were lower for Hythane™ fuelling. When compared on a carbon specific basis, however, natural gas hydrocarbon emissions were lower. At some test conditions, engine-out carbon monoxide (CO) emissions were lower under Hythane™. Engine-out oxides of nitrogen (NOx) were higher under Hythane™ fuelling for all test conditions when compared with natural gas at the same spark timing.Advancing spark timing (ST) from 20 °BTDC to 28 °BTDC increases engine efficiency slightly for both fuels. Engine-out NOx, increased with advanced spark timing at a greater rate under Hythane™ fuelling. An increase in THC emissions also occurred as spark timing was advanced with a similiar rate of increase for both fuels. Finally, CO emissions were almost completely independent of changes in spark timing.Fuel-air equivalence ratio (ϕ) was varied between 1.00 and 1.03. Maximum engine efficiency occurred at f values between 1.01 and 1.02 for both fuels. THC and CO emissions increased with richer mixtures while NOx emissions decreased. The rate of change of THC and NOx emissions with changing was greater under Hythane™ fuelling. CO dependence on was similiar for both fuels.Hydrocarbon speciation resulted in an exhaust gas hydrocarbon breakdown of 92% methane, 3% ethane, 3% ethene, 1.5% acetylene and trace quantities of other species. This result was consistent regardless of fuel type, engine speed or engine load.