Chromium(V) is an intermediate formed during the reduction of Cr(VI) to Cr(III) compounds by various bacteria. However, little is known about the nature, localization and reactivity of Cr(V) species in microbial systems. Electron paramagnetic resonance (EPR) spectroscopy was used to study the nature of Cr(V) complexes generated in basalt-inhabiting Gram-positive Arthrobacter oxydans bacteria after exposure to high concentrations of Cr(VI). Numerical simulations of the EPR spectroscopic data provide strong evidence for at least two different diolato-type oxoCr(V) complexes (I, g(iso)=1.9801; II, g(iso)=1.9796) involving bacterial cell wall macromolecules in the Cr(VI)-A. oxydans system. The relative concentrations of the two oxoCr(V)-diolato species differ when Cr(VI) is incubated with either untreated A. oxydans cells (I:II approximately 50:50) or lyophilized cells (I:II approximately 10:90). Based upon the magnitudes of the proton superhyperfine coupling constants ((1)H a(iso)) for species I and II, the EPR simulation model is unable to distinguish unambiguously whether the oxoCr(V)-diolato species are linear alkoxides or cyclic diols (carbohydrates). The oxygen-containing functional groups associated with teichoic acids are the most likely candidates for complexation with the Cr(V) ion.