Hydrogen-rich fuels, via lean operating ability, potentially offer both emission control opportunities and engine efficiency gains not possible with gasoline. With a single-cylinder engine, the emission characteristics of two types of hydrogen-rich fuels were evaluated.The first fuel contained various proportions of hydrogen and gasoline. The test objective was to determine whether increasing HEF, the fraction of total fuel energy supplied by hydrogen, would significantly reduce HC emission at ultra-lean conditions. While holding NOx emission comparable to a 0.4 g/mile equivalent, HC emission was reduced 50 percent by increasing HEF from 0.13 to 0.48. However, even with this maximum practical level of hydrogen enrichment, the resulting HC emission was not significantly different than values obtained with richer mixtures of gasoline and still far in excess of a 0.41 g/mile equivalent. Therefore, hydrogen enrichment does not appear to be a sufficient means of reducing HC emission.The second type of hydrogen-rich fuel was a simulated product of an on-vehicle hydrogen generator that reforms gasoline into a gaseous mixture of CO, H2, CH4, and various diluents. Fueling a single-cylinder engine totally on this mixture at an equivalence ratio = 0.55 resulted in NOx and HC emission levels well below those obtainable with gasoline and possibly consistent with the most stringent future vehicle standards. CO emission was slightly higher than values characteristic of lean operation with gasoline. Major drawbacks to this concept for reducing emissions are: 1) the requirement of precise mixture control, 2) the fuel economy penalties exacted for converting gasoline to hydrogen-rich mixtures, and 3) the power losses in naturally aspirated engines operating on gaseous fuels containing large diluent fractions.