The present invention relates to a process for preparing leather by (I) tanning a pelt with (a) an aldehyde, or (b) a bisulphite-blocked polyisocyanate; (II) optionally, retanning the pelt with (a) a polyaspartic acid, a salt of a polyaspartic acid, a polyaspartic acid anhydride, or a mixture thereof, and/or (b) a polyaspartamide; (III) dressing the resultant product by (a) bottoming the product with a polyurethane and one or more natural bottoming assistants, and (b) applying a polyurethane and/or polyesteramide finish; and (IV) optionally, aftertreating the resultant dressed leather with a leather preservative.