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Plans to ban MTBE (Methyl tertiary-Butyl Ether) have led to discussions on how the gasoline in California will be formulated without any backsliding in air quality. One possibility is to replace MTBE with isooctane. Exhaust emissions using California Phase II gasoline with MTBE, were compared to a gasoline where MTBE had been replaced with isooctane. Regulated, particulate, carbon dioxide, and PAH (polyaromatic hydrocarbons) emissions were measured at 22 °C temperature for 8 vehicles using the European cycle for year 2000 (ECE+EUDC). One of the vehicles was a GDI (Gasoline Direct Injection), one was a carbureted model without a catalytic converter, and the others were equipped with multi point fuel injection and catalytic converters. Results indicate that no major backsliding in air quality can be expected when replacing MTBE with isooctane. NOx (nitrogen oxides) emissions were reduced in all vehicle types. CO emissions increased in the vehicle without a catalytic converter.