For a rapid and specific assay of alginate, the isolated polysaccharide is split by methanolic hydrochloric acid, the reaction products are separated on silica gel plates and visualized by an uronic acid specific naphthoresorcinol-hydrochloride acid reagent. With the solvent ethylacetat/tert. butanol/water (68 + 23 + 9, v/v/v) the Rf-values of the hydrolysis compounds of alginate differ distinctly from those of the other uronic acids, which are constituents of commercial polysaccharides. The method had been tested successfully on a series of different foods (e.g. propyleneglycol alginate in beer).