The particular position occupied by the aromatic hydroxy compounds in the chemistry of substance possessing tannoid character is not only evidenced by the natural classification of the tannins, tannin derivatives, and decomposition products so far isolated and investigated, but also by other chemical behaviour shown by these substances. Meunier and Seyewetz [Footnote:_Collegium_, 1908, 315, 195.], for example, were able to show that phenol, _p_-aminophenol, chlorophenol, trinitrophenol, catechol, resorcinol, hydroquinone, monochlorohydroquinone, orcinol, pyrogallol, and gallotannic acid precipitate gelatine from its aqueous solution, that is, to a certain extent possess tanning properties. The author has extended this series somewhat and obtained the following results:-- Relative Behaviour Towards Substances Gelatine. Hide Powder. Pelt. Tribromophenol Slight ppte. Tans Surface tannage [Footnote: In alcoholic solution] _o_-Nitrophenol No ppte. " " Br-_o_-Nitrophenol Slight ppte. " " Tribromopyrogallic Ppte. " " acid Bromophloroglucinol " " No tannage Galloflavine Slight ppte. " " Bromosalicylic acid " " " Bromo-[Greek: b] " " Tans -naphthol [Footnote: In alcoholic solution] Rosolic acid " " " [Footnote: In alcoholic solution] Gallic acid No ppte. No tannage No tannage By the condensation of their sulphonic acids, it may be demonstrated experimentally how the tannoid properties of nearly every member of the series are intensified. Investigattion in this direction, however, has not been systematically undertaken, for which reason the author determined to examine this subject; but the enormous number of samples required, obtainable only with great difficulty during the war, made it impossible to conclude completely the researches in this field. What little has so far been done relatively to this subject should, when collected, indicate the way to be pursued in this wide field of investigation. What follows will hence comprise the conversion of a few of the most important members of this series of substances into their methylene-condensation products with a brief discussion of the qualitative and tannoid reactions of the latter. The didepside of phenolsulphonic acid is obtained by condensing carbomethoxyphenolsulphonic chloride with sodium phenolsulphonate in the presence of the calculated amount of caustic soda. A product of the composition CH_3.0.COO.C_6H_4SO_2.0.C_6H_4.SO_3Na is first obtained, which on saponification with soda yields the pure didepside-- HO.C_6H_4.SO_2.C_6H_4.SO_3.Na