An oxidizing liquid (20), such as hydrogen peroxide, is vaporized (18) and the vapor is used to deactivate nerve gas, blistering gas, or other biologically active substances such as pathogens, biotoxins, and prions. A second chemical compound (42) in vapor, mist, or fog form is used in conjunction with the oxidizing vapor. In one embodiment, the second chemical preconditions the biologically active substances to be deactivated more efficiently by the oxidizing vapor. In another embodiment, the second chemical boosts the reactivity of the oxidizing vapor. In another embodiment, the other chemical reacts with the oxidizing vapor to form an intermediate compound that deactivates at least some of the biologically active substances.