A new type of polyoxometalate (POM), one with a hydrogen dinitrate group associated with a d-electron-substituted polyoxometalate, Fe-III[H(ONO2)(2)]PWIIO395- (1), has been developed that catalyzes highly effective aerobic sulfoxidation (RR'S + 0.5 O-2 -> RR'SO) under ambient conditions (1 atm of air at room temperature). Comparison of the rates for aerobic sulfoxidation of the mustard simulant 2-chloroethyl ethyl sulfide (CEES) catalyzed by 1 and several other species reported to be catalysts for this class of reactions, including NO+, NO2, (NH4)(2)Ce-IV(NO3)6 and An(III)Cl(2)NO(3)(CH3CN) indicate that 1 is clearly the most effective catalyst. Conversions and selectivities for the desired sulfoxide decontamination product (CEESO in these studies) are both effectively quantitative. The low or nonexistent catalytic activity of Fe-II(NO)PWIIO396-, Fe(NO3)3 and HNO3 argues strongly that nitrosyl and nitrate derivatives of the Fe polyoxometalate and nitric acid are not species important in catalytic turnover. The techniques of single crystal X-ray diffraction, solution NMR, FTIR, TGA, DSC, cyclic voltarnmetry, elemental and wet chemical analyses were applied to the characterization of I and these are collectively consistent with the alpha-Keggin structure with a strongly associated hydrogen dinitrate group and a more weakly associated nitrate of crystallization. (c) 2005 Elsevier B.V. All rights reserved.