Supplementing the gasoline-air intake mixture of an internal combustion engine with a hydrogen-rich fuel is one means of achieving very lean operation and low NOx levels that promise to meet the 0.4 g/mi standard. Steady-state engine dynamometer tests were conducted with a specially modified multicylinder engine to determine if the improved lean operating ability of this engine would enhance the hydrogen-supplemented fuel approach. For these tests the hydrogen enrichment was accomplished with both bottled hydrogen and simulated hydrogen-generator products.The hydrogen requirement of this engine was comparable to that of a 1973 Chevrolet engine tested at the Jet Propulsion Laboratory of the California Institute of Technology. Thus, the experimental engine did not benefit the hydrogen-supplemented fuel approach. At the lean mixtures required for very low NOx, the hot-engine hydrocarbon mass emission level was seven times the target for 1978 (0.41 g/mi) and approximately 60% greater than the minimum baseline level. The carbon monoxide mass emission level was approximately five times as great for supplementation with the simulated generator products as for supplementation with bottled hydrogen, and double the target for the 1978 standard of 3.4 g/mi.