Described is the use of at least substantially storage-stable supersaturated and acidified aqueous solutions of mixtures of aluminum triformate and alkali-metal and/or alkaline-earth formates as a means of supplying a water-dissolved mineral tanning agent based on aluminum formate for the tanning of leather and/or hides using little or no chromium. The invention thus concerns, in particular, storage-stabilized supersaturated and acidified aqueous solutions of this kind which also contain small quantities of stabilizers, preferred stabilizers being 1,2,3,4-butanetetracarboxylic acid and/or xanthane gum. The invention also concerns the use of such stabilized supersaturated aqueous forms of aluminum triformate solutions for use as tanning auxiliaries with a fixing and/or tanning action in the tanning or finishing of leather and hides.