This study examines the behaviour of three types of humic acids (natural humic acids, regenerated humic acids, and sulphited humic acids) with regard to their capacity to be used as tans and retans. The study makes use of conditions set out in a previous study (See Determination of penetration and fixation curves of leather using humic derivatives). Additionally, subsequent complementary tests were carried out using other retans that are commonly used in the tanning industry in order to compare and improve on the physical properties assessed here