Natural phenolic substances have recently been recovered from brown seaweeds using various techniques. Particularly, subcritical water extraction (SWE) has been developed to replace conventional processes because of its sustainable properties. In addition, ionic liquids (IL) have recently been proved to have potential capabilities in extracting bioactive compounds from natural resources. This study explores the power of the IL-assisted SWE method (SWE + IL) in obtaining different phenolic compounds from the brown seaweed Saccharina japonica. The imidazolium-based IL 1-butyl-3-methylimidazolium tetrafluoroborate [C(4)C(1)Cim][BF4] was provided as a catalyst in the SWE system at different temperatures (100250 degrees C) and concentrations (0.25-1.00 M) to examine its thermal and quantitative effects. Solid-liquid extraction (SLE) and SWE were additionally provided as references for the considered method. High-performance liquid chromatography analysis of the SWE and SWE + IL extracts showed high contents of gallic, chlorogenic, gentisic, protocatechuic, p-hydroxybenzoic, and caffeic acids along with minor amounts of vanillic and syringic acids. The highest total content of phenolics was determined at 175 degrees C in two SWE techniques; however, in conventional SLE, phenolic compounds were poorly detected. Various antioxidant-determining assays such as 2,2-diphenyl-1-picrylhydrazyl radical scavenging assay, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical scavenging assay, total antioxidant capacity assay, and ferric reducing antioxidant power assay indicated the high antioxidant activity of extracting samples from SWE and SWE + IL; however, limited capacity was observed from conventional SLE. Correlation testing and principal component analysis indicated a strong connection between phenolic content and antioxidant activity, thereby demonstrating the high quality of phenolic bioactive extracts from brown seaweeds. (C) 2017 Elsevier B.V. All rights reserved.