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The oxygenation of 4′-substituted 1H-2-phenyl-3-hydroxy-4-oxoquinolines (PhquinH2) in a DMSO/H2O (50/50) solution leads to the cleavage products at the C2–C3 bond in about 75% yield at room temperature. The oxygenation, deduced from the product compositions, has two main pathways, one proceeding via an endoperoxide leading to CO-release, and the other through a 1,2-dioxetane intermediate without CO-loss. The reaction is specific base-catalyzed and the kinetic measurements resulted in the rate law [OH−] [PhquinH2] [O2]. The rate constant, activation enthalpy, and entropy at 303.16 K are as follows: ΔG ‡=73.13±4.02 kJ mol−1; ΔH ‡=70.60±4.04 kJ mol−1; ΔS ‡=−28±2 J mol−1 K−1. The reaction fits a Hammett linear free energy relationship for 4′-substituted substrates, and electron-releasing groups make the oxygenation reaction faster (ρ=−0.258). The EPR spectrum of the reaction mixtures showed the presence of the organic radical 1H-2-phenyl-3-oxyl-4-oxoquinoline and superoxide ion due to single electron transfer from the carbanion to dioxygen. The pathway via 1,2-dioxetane could be proved by chemiluminescence measurements. [ABSTRACT FROM AUTHOR]