Abstract: A process for the production of mineral tanned or/and re-tanned or/and after-treated leather or pelt, wherein before the mineral tanning, directly after the mineral tanning, directly before the mineral re-tanning, after the mineral re-tanning or/and with the after-treatment of a fatliquoring with a polyvalent metal cation, the substrate is treated with(A) a polymeric carboxylic acid containing .alpha.-hydroxyacrylic acid units, optionally in salt form, as constituent units of the polymer,by which a high exhaustion of mineral (re)tanning agent, an excellent distribution of the tanning agent and very pleasant soft handle of the (re)tanned leather resp. an outstanding water resistance of mineral after-treated fat-liquored leather can be achieved.