A large scale extract of the brown alga Fucus serratus was found to contain four arsenosugars (1–4) together with traces of dimethylarsinate (DMA) and arsenate by HPLC-ICP-MS. The extract was divided into 1500 equal subsamples which were then freeze-dried and stored at –18 °C. The homogeneity of this population of subsamples was established by random sampling and quantification of total arsenic by ICP-MS (1.22 µg As ±3.2%, mean ± RSD, n = 32) and hydride generation AAS (1.27 µg As ±2.0%, n = 32), and by quantification with HPLC-ICP-MS of the individual arsenic species in 10 subsamples: arsenosugar 1, 0.10 µg As ±3.6%; arsenosugar 2, 0.086 µg As ±2.9%; arsenosugar 3, 0.62 µg As ±3.8%; arsenosugar 4, 0.40 µg As ±3.1%; dimethylarsinate, 0.005 µg As ±20%; and arsenate ca. 0.001 µg As. The identity of the arsenosugars was confirmed by LC–electrospray (ES) MS with variable fragmenter voltage which provided simultaneous elemental and molecular detection. LC-ESMS was also used to quantify the four arsenosugars, producing values within 5% (2, 3 and 4) or 14% (1) of the ICP-MS data. The subsamples of Fucus extract provide a homogeneous source of natural material containing arsenosugars for use in arsenic speciation studies.