Binary cupric nitrate and triflate systems catalyze the homogeneous air oxidation of the mustard (HD) simulant 2-chloroethyl ethyl sulfide (CEES) to the corresponding desired sulfoxide (CEESO) with effectively quantitative selectivity in acetonitrile under ambient conditions. This activity is enhanced when cationic silica nanoparticles coated with the anionic multi-iron polyoxometalates (POMs) are also present. The POM-coated nanoparticles are prepared by treatment of aqueous suspensions of Akzo Nobel Bindzil CAT(R) cationic silica nanoparticles with aqueous solutions of the POMs, K-9[(Fe-III(OH2)(2))(3)(PW9O34)(2)] (K(9)4) or Na-12[(FeOH2)(2)Fe-2(P2W15O56)(2)](Na(12)5). (C) 2002 Elsevier Science B.V. All rights reserved.