We employed UV-vis spectroscopy to monitor real-time changes in the oxygen tension and concentration of reactive oxygen and nitrogen species (RONS) in deionized (DI) water during treatments with helium (He) and argon (Ar) gas plasma jets. He and Ar gas jets are both shown to de-oxygenate DI water with He being more efficient than Ar, whilst the plasma jets deliver and regulate the concentrations of hydrogen peroxide (H 2 O 2 ), nitrite (NO 2 - ) and nitrate (NO 3 - ) in DI water. The H 2 O 2 and NO 3 - production efficiency varied between He and Ar plasma jets, but was similar for NO 2 - . Whilst DI water fully equilibrated with ambient air prior to treatment (de-oxygenated by both plasma jets) when DI water was first de-oxygenated by an inert gas jet treatment, both plasma jets were found to be capable of oxygenating DI water. These insights were then used to show how different combinations of plasma jet and inert gas jet treatments can be used to modulate O 2 tension and RONS chemistry. Finally, potential further improvements to improve control in the use of plasma jets in regulating O 2 and RONS are discussed. © 2018 The Japan Society of Applied Physics.