This study aims at investigating the effect of methyl-tertiary butyl ether (MTBE) addition to gasoline on its octane number and, hence, the performance of an engine. Also, its effect on the emitted gases was investigated. Locally produced gasoline was blended with five different percentages of MTBE, namely 0%, 5%, 10%, 15% and 20%. Then, these fuels were burned in an engine, which is coupled to a gas analyzer. It was found that the octane number of the gasoline increases continuously and linearly with MTBE percentage in the gasoline, The best performance of the engine occurs at around 10% MTBE addition, and this percentage also gives the best reduction in exhaust gases emissions.