<p>The accumulation of nitrification products, specifically nitric acid (HNO₃) and nitrous acid (HNO₂), along with reactive oxygen species (ROS) such as H₂O₂, was investigated following cold atmospheric-pressure dielectric barrier discharge (DBD) plasma treatment of various liquid phases. Plasma generated by a planar DBD with a mesh electrode positioned above deionized water produced nitrification products without detectable accumulation of H₂O₂. In contrast, methanol and ethanol promoted H₂O₂ accumulation. Other organic solvents, including ethyl acetate, <i>t</i>-butanol, hexane, acetonitrile, dichloromethane and acetone, also yielded nitrification products, with most solvents exhibiting concurrent H₂O₂ formation, except acetonitrile. To further elucidate the role of liquid-phase composition, selected additives, including ROS quenchers, were introduced into aqueous systems to probe specific reaction pathways, and their effects on pH, oxidation–reduction potential, and nitrification were examined. Notably, hydroxyl radical scavengers showed negligible effects on nitrification in deionized water. In acidic aqueous solutions, L-glutathione (GSH) underwent S-nitrosylation to form GS-NO, whereas in 10% methanol-water mixtures, GSH was additionally converted into oxidized products, including GS-CH₂OH, GSO₃H, and GSSG. Phenol in deionized water was primarily converted into 4-nitrosophenol, 2-nitrophenol, and 4-nitrophenol, without detectable oxygenated products. Overall, these results indicate that liquid-phase composition modifies plasma–liquid chemistry and stable product distributions.</p>