Softening and waterproofing retanning agents

In the retanning of leather wherein the leather is contacted with a retanning agent, the improvement which comprises employing as said retanning agent the reaction product of

• (A) a terpolymer having molecular weights M.sub.n, determined as number average, of 5000 to 60 000 comprised of
• a) maleic anhydride,
• b) 80 to 120 mol %, relative to component a), of diisobutylene, and
• c) 1 to 12 mol % relative to component a), of at least one monomer of the formula ##STR3## in which R.sup.1 denotes hydrogen or methyl,
• R.sup.2 denotes hydrogen, methyl, ethyl, --OR.sup.3, ##STR4## unsubstituted or C.sub.1 -C.sub.4 -alkyl substituted phenyl or hydroxymethyl, and
• R.sup.3 denotes C.sub.1 -C.sub.4 -alkyl,
• (B) an aliphatic and/or cycloaliphatic monohydric C.sub.6 -C.sub.24 -alcohol, and
• (C) a base,
• wherein 20 to 80% of the carboxyl groups corresponding to the anhydride groups of the terpolymer are esterified with alcohol (B) and at least 20% of the remaining carboxyl groups are neutralized with base (C).