An auxiliary for the soaking of hides and skins, which consists of a salt of the formula IM.sup.+ [O.sub.3 S--S--R].sup.- Iwhere R is an organic radical of 1 to 8 carbon atoms, which may carry one or more functional groups, and M.sup.+ is one equivalent of a cation, mixtures thereof with other conventional soaking auxiliaries, and processes for their use. Using the novel auxiliary avoids all toxicity hazards and odor nuisance, as well as avoiding oxidation losses, and achieves good loosening of the hair.