The use of methyl tert-butyl ether (MTBE) as a fuel oxygenate is currently being phased out, with ethanol being the most likely replacement in most areas. Ethanol has been shown to have a negative influence on the biodegradation of petroleum hydrocarbons in the environment, with the possibility of increased benzene plume lengths. An experiment was conducted to determine the influence of ethanol and MTBE on the biodegradation and transport of benzene in fine-grained material (loess) from northern Illinois. Four undisturbed columns (0.3 m [1 ft] in diameter by 0.4 m [1.3 ft] long) of loess were collected. Uncontaminated groundwater was mixed with benzene, benzene plus MTBE, or benzene plus ethanol for injection into the column. A fourth column was continuously fed uncontaminated groundwater for 90 days as a control. Effluent concentrations of benzene were higher in the benzene-plus-ethanol and benzene-plus-MTBE column than in the benzene-only column, with the lower retardation of benzene in the presence of MTBE and ethanol. Aerobic ethanol biodegradation lowered the pH, increased the conductivity, and increased Fe(II) concentrations compared to the benzene-alone and benzene-plus-MTBE columns. This study indicates that benzene was biodegraded in the presence of ethanol, but at a substantially lower rate than benzene alone or in the presence of MTBE. The results of this study shows that the addition of ethanol or MTBE could pose a more serious threat of benzene contamination to groundwater supply wells because of slower degradation, larger benzene plumes, and decreased sorption.