To assess the effectiveness of aqueous SO2, in extracting metal values from discarded smelter slag, the dissolution behavior of Co, Ni, and Fe using SO2(aq) and H2SO4 was investigated. Experiments were carried out in batch mode under ambient conditions, with an emphasis on the relative contribution of acid (H+) and SO2(aq) complexation to metal extraction. The results suggest that both acid and sulfur(IV) ligand attack the reactive sites on the smelter slag surface to dissolve the metals. SO2(aq) was found to be more effective in leaching slag than H2SO4 at the same pH. The effect of acid was more predominant at lower pH, while the ligand effect of SO2(aq) became stronger at higher pH. The leaching process was modeled to quantify the relative contribution of these two dissolution mechanisms using a new approach, which involves combining the shrinking-core model with an adsorption model. The model fit the leaching curves of Co, Ni, and Fe well and adequately described the experimental data.