Nowadays, bioalcohols are promising renewable alternative fuels to gasoline in road transport. One of the most crucial issues of many alcohol fuels is limited information about their compatibility with fuel line materials such as steel. This study aims to evaluate the corrosion aggressiveness of alcohol–gasoline blends to mild steel and stainless steel. Two alcohol–gasoline blend types were tested: ethanol–gasoline and butanol–gasoline blends. These fuels were tested in pure (noncontaminated) and contaminated forms. The corrosion studies were performed using electrochemical impedance spectroscopy in a planar, two-electrode arrangement. We found out that stainless steel was highly corrosion resistant in all of the tested fuels, while mild steel exhibited very good corrosion resistance only in pure alcohol–gasoline blends. However, the corrosion resistance of mild steel decreased considerably after contamination, especially in the case of butanol–gasoline blends, and the highest corrosion rate of mild steel was measured in the contaminated B85 fuel with 6 vol % water having the lowest value of polarization resistance of 128 kΩ cm². Furthermore, pitting corrosion was observed in B40 and higher fuels even at short exposure times, and, once being contaminated, butanol–gasoline blends were found to be more aggressive to mild steel.