Complex basic zirconium salts and aluminum salts

In a process for tanning skins and hides by treating the delimed and pickled skins and hides with an acid aqueous solution of a mineral tanning agent at a pH which is from 4.2 to 4.6 at the end of the treatment, the improvement which comprises using as the mineral tanning agent a complex salt of empirical formula

• [(ZrO).sub.a (Al).sub.b (OH).sub.c (R).sub.d (X).sub.f ] H.sub.e ,
• in which R is a monocarboxylic acid radical of 1 to 4 carbon atoms, X is chloride or half of sulfate, a and b are independently of each other from 1 to 9, c is from 0 to ((4a+3b)-0.333(a+b)) and d is from 0.333 (a+b) to ((4a+3b)-0.333 (a+b)), provided c+d is from 0.333 (a+b) to (4a+3b), f is (4a+3b)-(c+ d) and e is 2a.