A novel double‐anion complex, H13[(CH3)4N]12[PNb12O40(VVO)2⋅(VIV4O12)2]⋅22 H2O (1), based on bicapped polyoxoniobate and tetranuclear polyoxovanadate was synthesized, characterized by routine techniques and used in the catalytic decontamination of chemical warfare agents. Under mild conditions, 1 catalyzes both hydrolysis of the nerve agent simulant, diethyl cyanophosphonate (DECP) and selective oxidation of the sulfur mustard simulant, 2‐chloroethyl ethyl sulfide (CEES). In the oxidative decontamination system 100 % CEES was transformed selectively to nontoxic 2‐chloroethyl ethyl sulfoxide and vinyl ethyl sulfoxide using nearly stoichiometric 3 % aqueous H2O2 with a turnover frequency (TOF) of 16 000 h−1. Importantly, the catalytic activity is maintained even after ten recycles and CEES is completely decontaminated in 3 mins without formation of the highly toxic sulfone by‐product. A three‐step oxidative mechanism is proposed.