A composition including an acetal of an aldehydic tanning agent for tanning hides and/or pelts, as well as a method for tanning hides and/or pelts in order to produce leather, wherein an acetal of an aldehydic tanning agent is used. By reducing the pH, the aldehydic tanning agent can be released, whereby a tanning step is initiated. The composition and the method are characterized by lower toxicity compared to conventional compositions and tanning methods based on aldehydic tanning agents. Furthermore, improved penetration of the tanning agents into deeper layers of the hide and/or the pelt is possible, so that particularly homogeneous tanning is achieved.