A study of natural gas as a low-pollution automotive fuel was conducted using a single-cylinder engine, a multicylinder engine, and six vehicles. Results showed that the light-load, lean-limit misfire region of natural gas begins at an air-fuel ratio between 140-150% of stoichiometric. Changes in ignition timing significantly influenced emissions of nitrogen oxides and hydrocarbons but had little effect on carbon monoxide emission. Lower emissions can be achieved (by adjustment) with current design engines, but with heavy penalty to engine performance. Emissions from vehicles fueled with natural gas are virtually unaffected by temperature change within the range 20-100 F.Natural gas exhaust is estimated to be 22-25% as reactive as gasoline exhaust.