The fuel supply chain faces challenges associated with microbial contamination symptoms. Microbial growth is an issue usually known to be associated with middle distillate fuels and biodiesel, however, incidents where microbial populations have been isolated from unleaded gasoline storage tanks have also been recently reported. Alcohols are employed as gasoline components and the use of these oxygenates is rising, especially ethanol, which can be a renewable alternative to gasoline, as well. Despite their alleged disinfectant properties, a number of field observations suggests that biodeterioration could be a potential issue in fuel systems handling ethanol-blended gasoline. For this reason, in this study, the effect of alcohols on microbial proliferation in unleaded gasoline fuel was assessed. Ethanol (EtOH), iso-propyl alcohol (IPA) and tert-butyl-alcohol (TBA) were evaluated as examples of alcohols utilized in gasoline as oxygenates. Two different commercial grades of unleaded gasoline were employed in the study, namely a standard (U) and a high octane grade (SU) according to European market nomenclature. The gasoline samples were blended with EtOH, IPA and TBA at various mixing ratios, the resulting blends were contaminated with uncharacterized "bottoms-water" of known microbial activity and the resulting microcosms were stored for a certain period of time. During storage the microbial growth was monitored by utilizing a quantitative microbiological method and alterations in some quality parameters of the stored fuel blends were also examined.