An anaerobically digested, municipal sewage sludge was mixed with an acid and a neutral soil and incubated for periods of up to 30 weeks at 25 ± 2°C and approximately −33 kPa water potential. Aliquots from saturation pastes were passed through a Sephadex™ G-15 gel filtration column and the fractions analyzed for total soluble C, Cu, Ni, and Zn. Infrared spectra were recorded for fractions containing a soluble-C or metal-concentration maximum. Copper was associated with soluble organic compounds throughout the 30-week incubation, and through 4 weeks was found primarily in fractions containing amide functional groups. Zinc generally eluted in a region suggesting the presence of Zn-inorganic complexes. Soluble Ni was associated with organic compounds at high pH during the initial 4 weeks of incubation, but in extracts of subsequent incubations the element partitioned about evenly between organic and inorganic complexes below neutral pH. The relative strength of adsorption bands in the IR spectra, which were diagnostic for polysaccharides, proteins, and amino acids, declined over the study period. The dominant spectral features for extracts of sludge-soil mixtures incubated 4 weeks or longer were diagnostic bands for aromatic and aliphatic carboxylic acids. A least squares (L2) approximation to proton titration curves was used to select a subset of ligands, which provided the smallest error term. The set of ligands from which the subset was selected was acetate, citrate, maleate, phthalate, salicylate, arginine, lysine, ornithine, and valine. Total soluble concentrations of Ca, Cu, Ni, Zn, Cl, NH3, PO4 and organic ligands, and pH and PCO2 values (0.03 kPa), were used to calculate ion speciation in the extracts via the computer program GEOCHEM. Values for the fractions of total soluble Cu, Ni, and Zn bound by soluble organic matter as predicted by GEOCHEM agreed with values determined by gel separation.