Benzene, toluene, ethylbenzene, para-, meta-, and ortho-xylenes, collectively referred to as BTEX, constitute some of the most environmentally detrimental organic compounds that have made their way into groundwater, primarily due to gasoline spills and underground rusted storage tank leakage over time. In this chapter, the author observes not only the six BTEX compounds, however; in addition, he observes a large and early eluting peak that matches the retention time of methyl-tert-butyl ether (MTBE). MTBE was used as a gasoline additive designed to boost octane rating and was thought at the time to be a suitable substitute for tetraethyl lead. BTEX compounds comprise about 20 to 30% of gasoline and have appreciable solubility in water in contrast to aliphatic hydrocarbons such as n-heptane, iso-octane, and n-dodecane. The Energy Policy Act of 2005 prompted gasoline refiners to transition away from MTBE to the use of ethanol as a gasoline additive.