These condensation products gained extraordinary importance for the tanning trade when Stiasny [Footnote: Ger. Pat, 262,558; Austr. Pat, 58,405.] succeeded in preparing them in water-soluble form when they are enabled to directly exert their tannoid properties. This may be done by acting upon two molecules of concentrated phenolsulphonic acid with one molecule of formaldehyde, the temperature thereby not exceeding 35°C. By condensation, however, considerable heat is liberated, and hence the rise in temperature can only be limited by adding the diluted formaldehyde drop by drop, whilst stirring and cooling, to the phenolsulphonic acid. The original letters patent is worded as follows: 10 kilos each of crude phenol and sulphuric acid (66° Bé.) are heated with stirring for two hours at 105°-106°C., cooled to about 35°C., and 463 kilos 30 per cent. formaldehyde added during three hours, the temperature thereby not exceeding 35°C.; the stirring is continued for a couple of hours after the final addition of formaldehyde. This yields about 24 kilos of the crude condensation product. On a commercial scale, however, cresol (cresylic acid) is substituted for phenol. There are three isomers of cresol, viz., _o_-, _m_-, and _p_-cresol, and it was naturally of interest to investigate whether one or the other of the isomers exerted any particular influence on the properties of the final product. It was found, however, that condensation products from the three isomers were distinguishable from one another neither in physical nor in tannoid properties. It is hence possible to employ crude cresol, which contains varying quantities of the _o_-, _m_-, and _p_-compounds, in the manufacture of these tanning matters. [Footnote: Gen Pat, 291,457.] The tar obtained from the Rochling coal-gas generator contains considerable quantities of phenols (B.P.=200°-250°C.), and the author has protected the use of these for the production of synthetic tannins by Ger. Pat, 262,558. A deep brown viscous mass is obtained which, when partly neutralised, yields similar results to those given by the product above referred to. It may be anticipated that by analogy from the chemical reactions taking place in the condensation of phenols on the one hand and cresolsulphonic acid on the other, that all other homologues of phenol, its polyvalent derivatives, substitution products and acids, would yield similar condensation products.