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In this work, the intensification of the extraction of flavonoids from dry onion skin wastes (DOSW) by Ultrasound Assisted Extraction (UAE) was investigated. Conventional stirred tank extraction was used to find the optimal temperature (37 °C) and solvent (ethanol 70%, v/v), providing 20.7 ± 0.4 mg QE/g DOSW after 1 h. In the UAE process, the amplitude of oscillation and the solvent-to-solid ratio were key parameters to control the specific energy used. Energies above 10 kJ/g OSW did not increase the extraction yield (maximum flavonoids content was 23.9 ± 0.2 mg QE/g DOSW), which could be obtained using amplitudes of 40%, high solvent-to-solid ratios (30 mL/g DOSW) and extraction times below 5 min. Although both extraction techniques provided similar profiles of phenolic compounds, being quercetin and quercetin-4′-O-glucoside the main flavonoids (7.4 mg/g DOSW and 10.4 mg/g DOSW, respectively), UAE increased the productivity up to 11.6 mg QE/(g DOSW·min), seven times higher than conventional extraction. The extraction kinetics was modeled and studied. The optimum extract was freeze-dried resulting in a solid powder rich in flavonoids (21%) which kept 90% of the antioxidant activity after 180 days of storage at temperatures below 4 °C. [ABSTRACT FROM AUTHOR]