Nonthermal plasma-induced decomposition of ortho, meta, and para isomers of hydroxy-, chloro-, and nitrophenols in water was investigated using the hybrid series gas-liquid electrical discharge reactor that generates gas-phase discharge above the water surface simultaneously with the electrical discharge directly in the liquid. Degradation of substituted phenols was evaluated for two gas-phase compositions, pure argon and pure oxygen, above the aqueous solution. Removal of all phenols was found to follow first-order kinetics. Hydroquinone was the most reactive among substituted phenols under an oxygen atmosphere, whereas in argon the most reactive were 4-chlorophenol and 2-nitrophenol. Electrophilic attack by hydroxyl radicals and ozone were determined to be the main oxidation pathways for degradation of phenols in the hybrid series reactor under argon and oxygen atmospheres, respectively. Hydroxylated aromatic byproducts were identified during degradation of all substituted phenols under both gas-phase compositions. In addition, cis,cis-muconic acid was detected during degradation of catechol under an oxygen atmosphere. An electrophilic substitution reaction mechanism was also proved by the significant correlation between the relative rates of oxidation of substituted phenols obtained in the hybrid series reactor and the Hammett substituent constants. Dechlorination and denitration of chlorophenols and nitrophenols, respectively, corresponded typically to about 50% of total conversion of parent compounds.