AIMS: Escherichia coli is able to reduce azo compounds such as methyl red (MR) and nitro compounds such as 7-nitrocoumarin-3-carboxylic acid (7NCCA). The aim of this study was to clarify the specificity of the major E.~coli reductases. METHODS AND RESULTS: Enzymatic assays with pure enzymes obtained after cloning, overproduction and purification under native or denaturing conditions were performed on three enzymes: AzoR, NfsA and NfsB. Their dependence on putative cofactors such as flavin mononucleotide (FMN), NADH and NADPH was studied as well as the reductase capacity of E.~coli mutants depleted for one, two or three of the corresponding genes. CONCLUSIONS: AzoR was able to reduce both MR and 7NCCA, whereas NfsA and NfsB could only reduce the nitro compound. AzoR and NfsB were strictly FMN dependent in contrast to NfsA. At a low oxygen concentration, the three proteins were not mandatory for azo reduction and nitro reduction, but in optimal aerobic conditions, azoR was essential for MR reduction, and an nfsA/nfsB combination was important for 7NCCA reduction. Overexpression of azoR gene was able to compensate for the loss of nfsA and nfsB under aerobic conditions. SIGNIFICANCE AND IMPACT OF STUDY: These data provide new insights into the substrate specificity of major E.~coli nitroreductases and demonstrate that oxygen is an important parameter to take into account in studies of nitroreductase activity.