Tanning-agents are obtained by condensing together, in an alkaline medium, (a) a cyclic sulphonic acid containing a hydroxyl group, (b) an unsulphonated hydroxy compound of the benzene series, and (c) an addition product of a sulphite and an aldehyde, a ketone, an aldehyde-alcohol, a keto-alcohol, an aldehyde-acid or a keto-acid, with or without (d) an organic sulphonic acid. The components of type (a) preferably contain a nuclear hydroxy group; the sulphonic group in the components of type (a) may be linked either to the nucleus or to an aliphatic side chain. Specified components of type (a) are (1) phenol- or cresol-sulphonic acids or their substitution products or their condensation products with aldehydes, ketones containing up to three carbon atoms, acetylene or sulphur; such condensation products may be made from the parent phenol or cresol, and the sulphonic group may be introduced before, during or after the condensation; specified sulphonated condensation products are di-(p-hydroxyphenyl)-dimethylmethane disulphonic acid, the products obtainable by addition of formaldehyde-bisulphite to phenol-acetaldehyde resins or to dihydroxy-diarylmethanes such as di(hydroxyphenyl)-dimethylmethane, sulphonated resinic residues from the purification of cresol, sulphonated phenol-acetylene resins, and condensation products from cresolsulphonic acid and formaldehyde; (2) sulphonation products of humic acids or of humous materials such as lignite or peat; the reaction products of humic acids with formaldehyde-bisulphite are particularly suitable; (3) lignin sulphonic acid. The components of type (b) may be phenols, their halogen, nitro or amino substitution products, or their condensation products with aldehydes such as formaldehyde or acetaldehyde or with ketones, acetylene or sulphur; specified components are phenol, cresols, xylenols, chlorophenols, resorcinol, dihydroxydiphenylmethanes, di-(p-hydroxyphenyl)-dimethylmethane, and alkali-soluble condensation products of phenol with formaldehyde, acetaldehyde, acetone, acetophenone, acetylene or sulphur. Specified components of type (c) are the addition products of sulphites with formaldehyde, acetaldehyde, chloral, aldol, glycerine aldehyde, acetone, pyroracemic acid, glucose, benzaldehyde, salicyl aldehyde, and acetophenone; the sulphite and the aldehyde or ketone may be added as such to the reaction mixture. Specified components of type (d) are benzenesulphonic acids, naphthalenesulphonic acids, anthracenesulphonic acids, phenolsulphonic acids, cresolsulphonic acids, the condensation products of these with aldehydes or ketones, the reaction products of formaldehyde-bisulphite with dihydroxydiaryl sulphones or dihydroxydiarylmethanes, and the products from formaldehyde-bisulphite and humous materials such as humous coals or peat. The amount of component (a) must exceed that of component (b). The reaction temperature may be 80--100 DEG C., and the pressure may be atmospheric or above. The reaction mixture may be made alkaline by means of calcium or barium hydroxide, and the products may be after-treated to eliminate calcium or barium. In the examples, (1) sulphite cellulose lye and cresol are heated in aqueous caustic soda with sodium bisulphite and formaldehyde; (2) sulphite cellulose lye and di - (p - hydroxyphenyl) - dimethylmethane (or a phenol-acetaldehyde condensation product) are heated in aqueous caustic soda with sodium bisulphite and formaldehyde; (3) sulphite cellulose lye and resorcinol are treated in alkaline solution with sodium bisulphite and formaldehyde; (4) sulphite cellulose lye and di - (p - hydroxyphenyl) - dimethylmethane are heated in aqueous alkaline solution with formaldehyde, and the product is further heated with sodium sulphite and formaldehyde; (5) an alkaline solution of sulphite cellulose lye and cresol is heated with sodium bisulphite and acetaldehyde or acetone; (6) an alkaline solution of sulphite cellulose lye and cresol is heated with glucose and sodium bisulphite or (example 7) with aldol and sodium bisulphite or (example 8) with salicyl aldehyde and sodium bisulphite; (9) a condensation product from cresolsulphonic acid (or b -naphthalenesulphonic acid) and formaldehyde is heated with cresol, sulphite cellulose lye and caustic soda are added, and the mixture is heated with sodium bisulphite and formaldehyde; (10) a condensation product from cresolsulphonic acid and formaldehyde is heated in aqueous caustic soda with cresol, formaldehyde and sodium bisulphite, and the mixture is further heated after addition of sulphite cellulose lye, formaldehyde, and sodium bisulphite; (11) a mixture of cresol and cresolsulphonic acid is heated in aqueous caustic soda with sulphite cellulose lye, formaldehyde, and sodium bisulphite; (12) a condensation product from cresolsulphonic acid and formaldehyde is heated in aqueous caustic soda with cresol and formaldehyde, and heating is continued after addition of sulphite cellulose lye, aldol, and sodium bisulphite; (13) a condensation product from formaldehyde-bisulphite and 4 : 4<1>-dihydroxydiphenyl sulphone is heated in aqueous caustic soda with 4 : 4<1>-dihydroxydiphenyldimethylmethane, and heating is continued after addition of sulphite cellulose lye, sodium bisulphite and formaldehyde; (14) a mixture of cresol and cresolsulphonic acid is heated in aqueous caustic soda with formaldehyde and sodium bisulphite; (15) a condensation product from cresolsulphonic acid and formaldehyde is heated in aqueous calcium hydroxide with cresol, formaldehyde and sodium bisulphite, and the product is freed from calcium by addition of sulphuric acid and ammonium sulphate; (16) a condensation product from 4 : 4<1>-dihydroxydiphenyl sulphone and formaldehyde-bisulphite is heated in aqueous caustic soda with cresol, formaldehyde and sodium bisulphite; (17) a condensation product from phenol-p-sulphonic acid and acetone is heated in aqueous caustic soda with cresol, formaldehyde and sodium bisulphite; (18) a condensation product from cresolsulphonic acid and formaldehyde is heated in aqueous caustic soda with 4 : 4<1>-dihydroxydiphenyldimethylmethane, formaldehyde and sodium bisulphite; (19) a condensation product from cresolsulphonic acid and formaldehyde is heated in aqueous caustic soda with cresol, aldol and sodium bisulphite.