A highly sensitive spectrophotometric method for the selective detection of catechol compounds such as catechol siderophores (e.g., enterobactin) is described. The basis of the method involves the ability of the vicinal aromatic hydroxyl groups under acidic conditions to bring about a reduction of Fe3+ (from ferric ammonium citrate) to Fe2+. Detection of Fe2+ in the presence of Fe3+ is made with 1,10-phenanthroline under previously established conditions. The assay mixture is heated at 60 degrees C for 1 h to accelerate the development of color which is subsequently measured at 510 nm. The Beer-Lambert law is obeyed over the range of 0.16 to 60 microM 2,3-dihydroxybenzoic acid. Compared to the Arnow nitration method, the assay is more responsive, is approximately seven times more sensitive, and is effective with catechols substituted at positions 3 and 4. The method gives positive results with catechols such as DL-DOPA, L-dopamine, (+/-)-epinephrine, and DL-norepinephrine. Very rapid color development is obtained with ascorbic acid and p-diols, while m-diols are poorly detected. Low degrees of reactivity are shown by hydroxylamino and hydroxamate compounds. Phenolic, sulfydryl, indolyl, and quinonyl derivatives do not interfere with the reaction. The method has been adapted to determine catechol compounds in the culture medium of bacterial cells grown at different iron concentrations.