Three cases of MTBE contamination of ground water are presented. Each case site had its own characteristics and different geochemical condition. The first case site was where dissolved oxygen was the dominant electron acceptor, while the sites of the second and third cases were under the anaerobic conditions where iron reduction and methanogenesis were the major biodegradation process of BTEX and MTBE. At the first case site, total mass of BTEX compounds decreased over the three sampling rounds, indicating a net loss in the aquifer. MTBE data did not show a clear trend. Under the anaerobic conditions of the second and third cases, the attenuation of MTBE could be evidenced from the MTBE plume distribution data and mass balance analysis. Through several years of study and field works at several sites under different geochemical conditions, we could collect evidence of the natural attenuation of MTBE in ground water. Even though the attenuation was not fast compared to those of BTEX compounds, it was noticeable and the rate could be estimated.