In order to enhance the efficiency of hydrodynamic cavitation, the addition of high purity water has been evaluated as a novel treatment scheme for thermal water treatment. The current work is the first report to outline a treatment strategy for thermal water, where high purity water was used as an active chemical for additional radical source. The increased cavitation efficiency observed with non-purgeable organic carbon and total nitrogen measurements can be linked to an enhanced concentration of pure water vapor filled submicrocavities as compared to those in bulk thermal water. The phenomenon appears to be closely related to the quality of cavities collapsing generated under the same condition but in the mixture of two different water phases.