AbstractIntroductionPhlorotannins are complex mixtures of phloroglucinol oligomers connected via C‐C (fucols) or C‐O‐C (phlorethols) linkages. Their uniformity in subunits and large molecular weight hamper their structural analysis. Despite its commercial relevance for alginate extraction, phlorotannins in Laminaria digitata have not been studied.ObjectiveTo obtain quantitative and structural information on phlorotannins in a methanolic extract from L. digitata.MethodologyThe combined use of 13C and 1H NMR spectroscopy allowed characterisation of linkage types and extract purity. The purity determined was used to calibrate the responses obtained with the colorimetric 2,4‐dimethoxybenzaldehyde (DMBA) and Folin–Ciocalteu (FC) assays. Using NP‐flash chromatography, phlorotannin fractions separated on oligomer size were obtained and enabled structural and molecular weight characterisation using ESI‐MS and MALDI‐TOF‐MS.ResultsThe fucol‐to‐phlorethol linkage ratio was 1:26 and the extract was 60.1% pure, determined by NMR spectroscopy. For DMBA, the response of the extract was 12 times lower than that of phloroglucinol, whereas there was no difference for FC. By accounting for differences in response, the colorimetric assays were applicable for quantification using phloroglucinol as a standard. The phlorotannin content was around 4.5% DM. Fucol‐ and phlorethol‐linkage types were annotated based on characteristic MSn fragmentations. Structural isomers of phlorotannins up to a degree of polymerisation of 18 (DP18) were annotated and identification of several isomers hinted at branched phloroglucinol oligomers. With MALDI‐TOF‐MS phlorotannins up to DP27 were annotated.ConclusionBy combining several analytical techniques, phlorotannins in L. digitata were quantified and characterised with respect to fucol‐to‐phlorethol linkage ratio, molecular weight (distribution), and occurrence of structural isomers. Copyright © 2017 John Wiley & Sons, Ltd.