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• Free and bound phenolics were identified by UPLC-Q-TOF-MS in hawthorn. • Thermal processing results in different changes of free and bound phenolics. • Thermal processing significantly affected antioxidant activities of hawthorn. • The phenolic with the greatest contribution to antioxidant activity were determined. Thermal processing is a traditional method for processing hawthorn into food or medicine. In this study, the compositions of free and bound phenolic compounds in raw hawthorn were analyzed by ultra-performance liquid chromatography quadrupole-time of flight mass spectrometry, and the effect of thermal processing on phenolics and antioxidant activity was determined. Among the phenolics identified in unheated hawthorn, 26 were soluble, while only 10 were insoluble-bound. Thermal processing caused a significant reduction in total soluble phenolics content, but an increase in total insoluble-bound phenolics (p < 0.05). Procyanidin B2 and epicatechin showed the largest decreases in content, and were not detected in well-cooked hawthorn. The antioxidant activity also clearly decreased, with the chlorogenic acid, procyanidin B2, hyperoside, and isoquercetin contents correlating significantly (p < 0.05) with antioxidant activity. In general, the effect of thermal processes on phenolics and antioxidant activity was dependent on the types of phenolics and processing conditions. [ABSTRACT FROM AUTHOR]