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This study aims to the isolation of acylphloroglucinol derivatives with potential antioxidant effect from the brown alga Zonaria tournefortii and to the suggestion of their structure-activity relationship (SAR). Fractionation and HPLC purification led to the isolation of 4 major metabolites that were screened for their antioxidant activity via five complementary methods. Results indicated that all the tested compounds proved to have potential antioxidant activities. In DPPH and ABTS methods, the acylphloroglucinol (1) and (2) were found to be the most active compounds with IC 50 values of 42.76 ± 0.26 and 50.46 ± 0.12 µg cm -3 in DPPH assay and 12.04 ± 1.60 and 12.73 ± 1.47 µg cm -3 in ABTS assay, respectively. In contrast, in β-carotene and CUPRAC methods, the chromone derivatives (3) and ( 4) have revealed the high antioxidant activities with IC 50 values of 23.76 ± 0.63 and 20.99 ± 2.70 µg cm -3 in β-carotene assay and 19.06 ± 0.11 and 17.72 ± 0.26 µg cm -3 in CUPRAC assay , respectively. However, in metal chelating method all the investigated compounds demonstrated potent ferrous ion-chelating capability, with approximately the same level of activity.