This study shows that Cu occurs predominantly as Cu-sulfides and Cu-bearing phosphates and -carbonates in organic matter (OM) colloids within smelter-impacted soils in which they are often associated with other inorganic components. Major emitters of Cu are smelters and coal-power plants, which cause severe damage to the health of soils and aquatic systems as elevated Cu concentrations are toxic for terrestrial and aquatic organisms. Toxic effects and the long-term environmental fate of Cu depend among many other factors on its speciation in soil and water bodies. This study explores the role of OM colloids (defined as particles with diameters in the range of 100 to 1000 nm and with a larger proportion of organic than inorganic material) in the sequestration of Cu in contaminated soils around the Horne smelter, Rouyn Noranda, Quebec, Canada, focusing on a thin soil overlying bedrock (bedrock soil) and forest soil. The sequestration and mobilization of Cu by OM colloids are studied using a combination of column leaching experiments, ultra-centrifugation and transmission electron microscopy (TEM). TEM analysis indicates that Cu occurs as nano-sized CuSx phases in OM colloids of the bedrock soil, and as Cu-bearing Ca-Mg-phosphates and Ca-carbonates in OM colloids of the forest soil. The nano-sized CuSx phases occur along the rim of OM colloids or are attached to silica inclusions located within OM colloids, suggesting that their in situ formation is strongly controlled by the presence of polar groups within or on the surface of OM colloids. The proportion of Cu-bearing colloids in the soil leachates ranges from 20 to 40% of the total colloidal fraction, suggesting that OM colloids can play a significant role in the sequestration of Cu in surficial soil environments.