The effects of fuel formulation changes on vehicles meeting European Stage 1 (91/441/EEC) and Stage II (94/12/EC) emission limits have been investigated. Vehicles in the Euro Stage II fleet were advanced specification versions of the vehicle models in the Euro Stage I fleet. However, the basic engine blocks and capacity were the same. The observed improvements in emissions were attributed to changes, such as position of the catalyst and lambda sensor, improved fuel delivery systems, and to improvements in engine control strategy. These engine modifications resulted in reduced catalyst light-off times and improved AFR control. Emissions improvements, over the modified European test cycle, as a result of these changes were approximately 50% for CO and NOx and 30% for THC.A fuel matrix was designed in order to study the effect of six fuel parameters on exhaust emissions from the two levels of vehicle technology. The first fuel in the series contained high aromatics, olefins and sulphur, no MTBE and had low mid-range and back-end volatilities. The sequential changes in individual fuel properties resulted in a significant change in fuel composition and distillation across the test matrix with the final fuel in the series being in effect a severely reformulated gasoline. The relative sensitivity of both vehicle fleets to changes in each fuel parameter was approximately the same. The changes in emissions attributable to fuel composition were smaller than those observed for improvement in vehicle technology.