Exposing water to 254 nanometer wavelength UV light can further purify it by sterilizing bacteria, thereby preventing their uncontrolled growth. 185 nanometer UV light oxidizes organic compounds, breaking them down into components such as CO2, which can then be removed by the ion exchange resins.

Since a 185 nanometer UV lamp actually produces light that is about 10% 185 nanometer and 90% 254 nanometer wavelength, it serves a dual purpose and also sterilizes any bacteria that pass through it. For design purposes, a UV oxidizer/sterilizer is usually located between two DI modules, so that the second DI module can polish the water back up to 18 megohm-cm, by removing the CO2, etc., generated by the oxidizing process.

A UV sterilizer is usually located after the final DI module, to ensure that bacteria cannot reproduce beyond that point. The use of a UV sterilizer after the final DI module, and before an ultrafilter module, can preclude the need for frequent, periodic chemical sanitization of the system.