This study aims at investigating experimentally the effects of methyl tert-butyl ether (MTBE)-gasoline blends on the performance and CO emissions of a spark ignition engine. The blends are prepared by blending 5, 10, 15, and 20 vol % of MTBE with a specified amount of base gasoline. Base, leaded, and unleaded gasolines are also used in the study. The experiments were conducted under various engine speeds, spark timings, and compression ratios. The engine was operated under wide open throttle conditions. The results of the study show that a 10 vol % of MTBE addition to the base gasoline yields the best engine performance and the least CO emissions while the blend containing 15% MTBE and 85% base gasoline suggests the best performance in terms of the brake thermal efficiency.