The leaching of heavy metals, such as copper, from municipal solid waste incinerator (MSWI) bottom ash is a concern in many countries and may inhibit the beneficial reuse of this secondary material. The enhanced leaching of copper from three MSWI bottom ash samples by dissolved organic carbon (DOC) was investigated with specific attention for the nature of the organic ligands. A competitive ligand exchange-solvent extraction (CLE-SE) method was used to measure Cu binding to DOC. Two types of binding sites for Cu were identified and geochemical modeling showed that the organically bound fraction varied from 82% to 100% between pH 6.6 and 10.6. Model calculations showed that complexation by previously identified aliphatic and aromatic acids was unable to explain the enhanced Cu leaching from the MSWI residues. High-performance size-exclusion chromatography (HPSEC) and the standard extraction procedure to isolate and purify natural organic matter revealed that about 0.5% of DOC consists of humic acids and 14.3-25.6% consists of fulvic acids. Calculated Cu binding isotherms based on these natural organic compounds, and the nonideal competitive adsorption-Donnan (NICA-Donnan) model, provide an adequate description of the organic Cu complexation in the bottom ash leachates. The results show that fulvic acid-type components exist in MSWI bottom ash leachates and are likely responsible for the generally observed enhanced Cu leaching from these residues. These findings enable the use of geochemical speciation programs, which include models and intrinsic parameters for metal binding to natural organic matter, to predict Cu leaching from this widely produced waste material under variable environmental conditions (e.g., pH, ionic strength, and concentrations of competing metals). The identified role of fulvic acids in the leaching of Cu and possibly other heavy metals can also be used in the development of techniques to improve the environmental quality of MSWI bottom ash.