X-ray absorption spectroscopy ͑XAS͒ was used to study the electrochemical incorporation of perrhenate anion, from a 0.1 M NH 4 ReO 4 solution, into poly͑vinylferrocene͒ ͑PVFc͒ and a modified PVFc ͑a copolymer of 30% t-butyl acetylferrocene and 70% t-butyl divinyl ferrocene͒. The polymers were deposited on a carbon cloth current collector from a solution of the polymers in CH 2 Cl 2 . In situ XAS measurements were done at the Fe K edge on the reduced polymers and at 0.9 V vs. Ag/AgCl. Ex situ XAS was done at the Re L 3 edge after oxidation of the polymers at 0.9 V. The oxidized electrodes were washed in water to remove dissolved NH 4 ReO 4 in the electrode pores. XAS was done both on wet-washed electrodes and on electrodes that were dried. XAS showed that at 0.9 V the Fe was oxidized from a ferrocene to a ferrocenium moiety and the FeuC bond distance increased from 2.05 to 2.08 Å. Both the X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure ͑EXAFS͒ results are consistent with having Ͼ75% of the ferrocene moieties in the polymer oxidized at 0.9 V. The Fe K-edge EXAFS showed no direct indication of interaction of Fe with ReO 4Ϫ . At the Re L 3 edge the only indication of interaction of ReO 4 Ϫ with the polymers was a slight change in the XANES features.Many industrial contaminants exist as anions. These include high-priority pollutants such as chromate, pertechnetate, and nitrate ions. Technetium is an important and difficult pollutant in nuclear waste. Because of its half-life of 213,000 years, technetium ( 99 Tc) presents a long-term hazard for waste disposal. Much of the 99 Tc in the tank wastes is present as pertechnetate (TcO 4 Ϫ ), accounting for its high solubility and mobility in aqueous systems. Conventional sorbents such as commercial ion exchange resins can be used to extract TcO 4Ϫ . However chemical elution of the ion exchange resin generates unwanted secondary wastes. An attractive alternative is to use an electrically conducting polymer ͑e.g., polyvinyl ferrocene, PVFc͒ to separate technetium from nitrate in high-level liquid nuclear waste. Absorption and elution can be done electrochemically without the generation of secondary wastes. The work reported here involves the use of in situ X-ray absorption spectroscopy ͑XAS͒ to study the interaction of anions with conductive polymer poly͑vinyl-ferrocene͒ electrodes. XAS investigation on pure ferrocene and related compounds has been reported earlier by others. [1][2][3][4] In this study perrhenate salts were used as nonradioactive surrogates for pertechnetate. The chemistry and electrochemistry of rhenium and technetium are rather similar. 5 The incorporation of anions in poly͑vinylferrocene͒ ͑PVFc͒ has been extensively studied. This work has been summarized in recent publications. 6,7 Until now the most effective in situ tool has been the electrochemical quartz crystal microbalance ͑EQCM͒. The technique is very effective in studies of anion intercalation and cointercalation of solvent. 6,7 However, EQCM does not yield chem...