Direct injection graphite furnace atomic absorption spectrometry (GFAAS) and hydride generation GFAAS techniques have been used to quantify As species in the National Research Council of Canada river water reference material SLRS-1 following their concentration and separation on a strong cation-exchange resin. Arsenic(III), AsV and dimethylarsinic acid account for 71% of the total As. Negligible amounts (<5%) of arsenocholine, tetramethylarsenic, monomethylarsonic acid and organically bound As were present. A significant fraction of the As (ca. 22%) exists as a relatively inert, unidentified species, which has similar but not identical properties to arsenobetaine. The species integrity of the sample has remained unaltered for the past 13 months of storage.