ntroduction - Phenolic compounds are metabolites exhibited at high levels in Phaeophyceae. Although several studies have been conducted on total phenol contents, no one to our knowledge has dealt with the contents of phenolic compounds and antioxidant activities on purified fractions. Objective - The purpose of this study was the extraction and purification of phenolic compounds from the brown seaweed Ascophyillum nodosum, to determine both their distribution and their radical-scavenging activities, and to obtain a sufficiently purified oligophenolic fraction to perform an RP-HPLC analysis on molecules with a molecular weight (MW) < 2 kDa. Methodology - Phenolic compounds were separated and purified by liquid-liquid extraction, tangential ultrafiltration and dialysis. Then, the contents of both phenolic compounds and radical-scavenging activities were measured by the Folin-Ciocalteu reagent, and DPPH and ABTS assays. NMR analysis was performed to validate the process. RP-HPLC with a C(18) column was performed on the oligophenolic fraction, using a novel method developed in this study. Results - Seven fractions were obtained as a function of polarity and molecular weight. Among them, the fraction containing phenolic compounds with a MW >= 50 kDa appeared to be the most active, correlated with the content of phenolic compounds. Conclusion - This work constitutes a step forward in the separation and purification of bioactive phlorotannins and represents a prerequisite for further investigations into their structural characterisation and distribution in A. nodosum.