The EPEFE study (European Programme on Emissions, Fuels and Engine Technologies), /1/ and other programmes have identified an increase in tailpipe NOx emissions with reduced gasoline aromatics content for modern 3-way controlled catalyst vehicles. This effect occurs with fully warmed-up catalyst under closed-loop operation. In order to understand the reasons for this effect VW and Shell have mechanistically investigated the effects of fuel properties on EMS (engine management system) and catalyst performance. Fuels with independent variation of oxygen, aromatics and mid-range volatility were tested in different VW engines. λ was monitored using sensors located both pre and post catalyst.The results confirmed that reducing gasoline aromatics content reduced engine-out emissions but increased tailpipe NOx emissions. It could be shown that differences in H/C ratio led to differences in the hydrogen content of engine-out emissions which affected the reading of the λ sensor. Reducing aromatics resulted in a lean shift in λ with the consequence of a reduced NOx conversion particularly at operating conditions which were already calibrated slightly lean. Activating the feedback control by the second sensor as used for OBD II could effectively compensate for the λ shift effect and resulted in a considerably reduced fuel sensitivity. Therefore with activated feedback control all regulated emissions were reduced over the composite European driving cycle with reduced fuel aromatics content due to lower pre light-off emissions.