Abstract: The adsorption of trace quantities of Cu (3·89 × 10−6 to 3·00 × 10−4 M) by a bentonite clay from calcium acetate solution was studied over the range of pH 4·27–5·87. The data were fitted to an existing adsorption equation and the ‘best’ values of the adsorption parameters were calculated. The proton, cupric ion and mono (hydroxy) cupric complex were found to be the adsorbed species. The adsorption sites are of two types. It has been postulated that the majority of sites are lattice hydroxyl groups located at crystal defects and edges. The remainder arise from organic matter adsorbed by the clay and are the more important in the adsorption of Cu at very low metal concentration and at pH <5·4. The implication of the results on the potential use of bentonite clay to remove trace amounts of Cu from mine waste waters is considered briefly.