This study examines the behavior of three different types of humic acids (natural humic acids, regenerated humic acids, and sulphited humic acids) with regard to their penetration and fixation to the leather. More specifically, through the tests carried out the curves of penetration and fixation were drawn, and from these the optimum pH intervals of penetration and fixation to the leather were determined. The data obtained is essential in view of the subsequent application of humic acids as tanning agents and/or retanning agents