The author's investigations of the electro-osmosis of an aqueous solution of Neradol D [Footnote: _Collegium_, 1920, 597, 24.] proved that dicresylmethanedisulphonic acid exhibits anodic migration; hence this product possesses negative charge and acidic character. The impurities accompanying the synthetic tannin, _i.e._, salts, free sulphuric acid, and some phenols, migrated anodic and cathodic respectively, according to their charges. A Neradol D purified by electro-osmosis finally yielded a pure solution of dicresylmethanedisulphonic acid, which precipitated gelatine and exhibited pronounced tanning effects, but gave a greenish-black coloration with iron salts. This conclusively proves that the blue coloration given by Neradol D with iron salts is no characteristic feature of the _pure_ synthetic tannin, but is caused by the phenolic impurities accompanying the latter. Especially the first stage of the electro-osmosis produces a cathodic migration of the phenols, which may then be detected at a cathode by means of the iron and bromine reactions. It is characteristic of a dicresylmethanedisulphonic acid purified by electro-osmosis that it does not precipitate aniline hydrochloride. It appears, therefore, that this reaction--which is characteristic of most synthetic tannins--is again caused by the presence of impurities. 3. The Influence of Salts and Acid Contents on the Tanning Effect of Neradol D Chemical analysis of crude Neradol revealed a natural dicresylmethanedisulphonic acid (the tanning agent) contents of about 68 per cent, which agrees fairly well with the calculated amount. Like other "strong" and "weak" acids this sulphonic acid exercises a strongly swelling influence on pelt. Whereas the effect of acid present in solutions of Neradol D of medium concentration and its tanning effect both influence the pelt and are fairly well balanced, this is not the case as regards highly concentrated and very dilute solutions. If, for instance, a very dilute solution of crude Neradol (about 0.25° Bé.) is used, the tanning effect of this solution is exceedingly small and does not show itself till after several hours. The relatively high dissociation of the acids at this high degree of dilution causes an extremely rapid and strong swelling of the pelt, which has therefore absorbed its maximum amount of water (maximum swelling) before the tanning effect of the sulphonic acid comes into play and by fixing the surface of the pelt is enabled to prevent the excessive swelling effect of the acids.