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Halogenated synthetic organic compounds are widespread contaminants of the environment; however, little is known concerning their potential dehalogenation by extracellular corrinoids. Our primary objective was to determine if corrinoids sorbed to sod colloids are redox active and thus viable electron transfer mediators of contaminant dehalogenation in reduced soils and sediments. Dicyanocobinamide, cyanocobalamin (vitamin [B.sub.12]), and aquocobalamin were sorbed onto [Ca.sup.2+]-, [K.sup.+]-, and [Na.sup.+]-hectorite and sorption isotherms determined. Additional assessment of sorption reactions was performed using x-ray diffraction (XRD) and infrared (IR) spectroscopy. Redox states of the sorbed corrinoids in hectorite suspensions were monitored using ultraviolet and visible (UV-VIS) spectrophotometry. Corrinoid molecular size and charge, as well as the saturating cation, controlled corrinoid sorption. The XRD results indicated that Co corrinoids intercalated hectorite, expanding the basal spacing from 1.45 to 2.21 nm for [Ca.sup.2+]-, 1.17 to 2.70 nm for [K.sup.+]-, and 1.31 to 2.81 nm for [Na.sup.+]-hectorite. The IR spectra of bound corrinoids yielded little information on binding mechanisms, but indicated that axial ligands remained intact. The central metal of bound corrinoids was reduced to Co(I) by Ti(III) and reoxidized to Co(III) by introduction of [O.sub.2], thus demonstrating that redox activity of sorbed corrinoids was maintained and reversible. Microbially produced extracellular corrinoids may act as potential electron transfer mediators when in association with clays, possibly participating in the reductive dehalogenation of organic contaminants.