Chlorine dioxide solutions are stabilized and prepared for storage, transportation, and testing. Each solution is separated into two samples. Chlorine dioxide is removed from one sample and then each sample is stabilized and prepared such that each sample contains only oxidized and/or reduced forms of chlorine dioxide. The stable samples may be stored, transported, and tested for chlorine dioxide. The samples are tested for the oxidized and/or reduced forms of chlorine dioxide by known methods, and mass balance equations are used to determine the concentration of chlorine dioxide in the original sample before stabilization and preparation.