In the course of many vehicle-based studies, it has become evident that a reduction in the aromatic content of gasoline fuels can result in an increase in tail-pipe NOx from catalyst-equipped vehicles (the ’NOx/aromatics’ effect). A laboratory-based study with simulated exhaust gas has been undertaken to investigate the cause of this effect. The results indicate that aromatic exhaust components, unlike short alkane exhaust components, act in a manner consistent with a range of other unsaturated hydrocarbons in contributing to the nitric oxide conversion over the catalyst. On the other hand, methane, and to a lesser extent ethane, are particularly unreactive and cause substantial breakthrough of nitric oxide from the catalysts. Vehicle-based studies have shown that these low reactivity hydrocarbons an predominantly produced by non-aromatic fuel components. Thus, reducing the aromatic content of fuels by substitution with alkanes and alkenes, will lead to a consequent reduction in nitric oxide conversion efficiency over the catalyst. Together, these results offer convincing evidence that methane generated during combustion of low aromatics fuels is the cause, wholly or in part, of the poor nitric oxide conversion efficiency observed when catalyst equipped cars are operated with these fuels.