Total metal content in the soil in itself is insufficient as a measure to indicate the actual environmental risks involved with soil contamination. Understanding the mobility of heavy metals in the soil and their speciation in the soil solution is of great importance for accurately assessing environmental risks posed by these metals. Solid-solution partition coefficients (soil-water distribution coefficient [K d]) for copper (Cu) and lead (Pb) were analyzed in a range of clean and contaminated agricultural soils, and metal speciation in soil solution was assessed. The influence of variables such as pH, soil organic carbon, clay content, and total soil metal concentration on the solid-solution partitioning and speciation of Cu and Pb were evaluated using multiple regression models. The best models were obtained for Kd that are based on total metal concentrations in both the soil solid phase, and the pore water and the various metal binding phases. In general, between 70% and 90% of the total variance was explained. The explained variance was significantly increased by taking the activity of the free metal into account. Strong correlations (R 2 = 0.88) were obtained by multiple regression analysis involving free metal ion (Cu 2+, Pb 2+) concentrations calculated with the equilibrium speciation model, Windermere Humic Aqueous Model/Model VI. Copyright © 2009 by Lippincott Williams & Wilkins, Inc.