Four siderophores isolated from bacteria have been studied as possible corrosion inhibitors for iron in deareated 1N HCl. These compounds are aerobactin, enterobactin, parabactin, and rhodotorulic acid. Polarization measurements and ac impedance studies show that all four compounds are inhibitors, with the order of effectiveness being rhodotorulic acid < aerobactin < enterobactin < parabactin. X-ray photoelectron spectroscopy surface analysis with one of the siderophores, rhodotorulic acid, shows that it chemisorbs at the iron/HCl interface. Enterobactin and parabactin are the most effective corrosion inhibitors of the four because they have the optimum combination of relatively high values of pK(al) and the largest molecular cross-sectional areas.