<p>This study investigated the adsorption of Cu²⁺ and Pb²⁺ onto a histosol (peat soil) as a function of pH and compared it to their adsorption onto its humin fraction. The use of the NICA-Donnan model, considering humic acid and humin as soil reactive solid phases and fulvic acid as reactive dissolved organic matter, satisfactorily described the adsorption behaviour of Cu²⁺ and Pb²⁺ onto histosol. Both metals were adsorbed mainly via carboxylic sites of both solid phases with a higher contribution of humic acid. Nevertheless, this study highlighted the significant role of humin, accounting for up to 37% of the overall metallic cation retention onto histosol. Thus, if not the most reactive solid organic matter fraction in soils, humin contributes notably to metal retention. Its contribution should thus be considered as a significant solid organic matter component of soils for a better description and prediction of metal trace element adsorption.</p>