An acknowledged consequence of utilising oxygenates such as MTBE as a gasoline component is known to be a lowering of CO exhaust emissions from mature technology vehicles due to the “natural” leaning effect that the inclusion of MTBE can provide. A small decrease in THC is also commonly seen in these circumstances, while the effect of MTBE on NOx emissions is more variable and not usually beneficial.The present paper describes the results of recent studies in the European arena, covering the effects of fuel oxygenates (notably MTBE) on regulated emissions for non-catalyst and catalyst car fleets examined in in-house programmes. It looks at emissions effects according to the broad classification of the onboard vehicle technology employed. It further cites experimental work that has featured MTBE replacement in gasolines by a single saturated hydrocarbon (2,3-dimethyl butane) that is isoelectronic with MTBE. Some related work conducted concurrently on splashblending is also described.The evidence from this and related work from other sources (e.g. the AQIRP study) is brought together to generate a view of the factors most important in determining the effects that MTBE inclusion as a fuel component can be expected to have, and how this may be affected by engine/vehicle technology. Evidence based in particular on the direct comparison of MTBE and 2,3-dimethyl butane effects (where the other fuel properties are effectively held constant) indicates that the classical emissions benefits of gasoline MTBE become less pronounced as engine/vehicle technology becomes more sophisticated.