The study reported herein indicated the stabilization mechanisms at work when copper-laden sludge is thermally treated with gamma-alumina and kaolinite precursors, and evaluated the prolonged leachability of their product phases. Four copper-containing phases - copper oxide (CuO), cuprous oxide (Cu2O), copper aluminate spinel (CuAl2O4), and cuprous aluminate delafossite (CuAlO2) - were found in the thermal reactions of the investigated systems. These phases were independently synthesized for leaching by 0.1 M HCl aqueous solution, and the relative leachabilities were found to be CuAl2O4 < CuAlO2 << Cu2O < CuO. The sintering condition and formation mechanism employed to stabilize copper into CuAl2O4 and CuAlO2 are extensively discussed here. With a 3 h of short sintering, it was found that CuAl2O4 could be effectively formed between 850 and 950 degrees C by the gamma-alumina precursor. Although kaolinite had a lower incorporation capability than gamma-alumina, it was found to transform a considerable amount of copper into CuAl2O4 between 950 and 1000 degrees C. At higher temperatures, CuAlO2 was produced only in the gamma-alumina system as the occurrence of Cu2O-cristobalite solution in the kaolinite system precluded the production of CuAlO2. The hypothesis that the spinel formation mechanism has two stages was supported by the results of the changing Cu/Al mole ratio in the system, and the rate-limiting step was identified as the diffusion process in the second stage. (C) 2010 Elsevier B.V. All rights reserved.