The present study explored the influence of Cl−, Br−, CO32−, HCO3−, PO43−, HPO42−, NO3−, SO32− and natural organic matter (NOM) on the reaction kinetics and the formation of undesired degradation byproducts during phenol oxidation by heat-activated persulfate (PS). CO32− and PO43− promoted the phenol degradation, because the hydrolysis of CO32− and PO43− created basic pH conditions which were conducive to enhanced PS oxidation rate. Br− promoted the reaction by reacting with sulfate radicals (SO4•−) to produce bromine radicals that can selectively react with electron-rich phenol. NOM scavenged reactive SO4•−, thus inhibiting the reaction. As a strong reducing agent, SO32− rapidly reduced PS, thus completely suppressing the reaction. HCO3−, HPO42−, Cl−, and NO3− had negligible impact on PS oxidation of phenol. Six intermediates were detected in the no anion control using gas chromatography–mass spectrometry (GC-MS). Various toxic halogenated phenols and halogenated hydroquinones were detected in the treatment containing Cl− and Br−. In contrast, in the treatment containing CO32−, HCO3−, PO43−, HPO42−, and NO3−, no new intermediates were identified except for the intermediates already detected in the control treatment. Based on intermediates identified, reaction pathways for PS oxidation of phenol without anions and in the presence of halides were proposed respectively.