The invention provides a method for generating a homogeneous aqueous mist solution containing a solvent such as water and a biocide agent such as chlorine dioxide, which would otherwise be unstable. The unstable biocide agent or chlorine dioxide is quickly dissolved or mixed with a mist of solvent causing the biocide agent to co-exist or co-mist therewith. The mist microencapsulates the biocide gas so that it does not decompose in the fumigation volume or space. The resulting homogenous mist solution provides a mist for delivering the biocide agent in a chemically stable form.Methods for mixing the separately generated mist and biocide gas include combining the mists in a Y-tube and then mixing the combination in a baffled mixing chamber, combining the mists in an area above their points of generation and then further mixing, and providing a series of mist generation units connected by a conduit for a carrier medium to pass to and connect the units and cause the mists to combine. Alternatively, the biocide may be released from a controlled source to dissolve in a generated solvent mist. Such controlled release of biocide includes providing a source of slow diffusion of chlorine dioxide gas to dissolve in water mist by convection and mixing, providing a continuous flow of chlorine dioxide solution via a coil reservoir for controlled contact with the mist, providing chlorine dioxide gas released from a solid-state mixture.