A fly ash-based stabilization/solidification (S/S) technique was investigated using field soil samples contaminated with arsenic (As) and lead (Pb). A semi-dynamic leaching test was used to evaluate the effectiveness of the S/S treatment. By assessing the cumulative fractions of leached As and pb, the effective diffusion coefficient (D-e) and a leachability index (LX) were measured and used for evaluating the effectiveness of the S/S treatment. Overall, As release was reduced by 98.3% and Pb release was reduced by 98.5% upon addition of 25% Class C fly ash. The mean D-e decreased significantly and the mean LX was always above 9 for all treated samples, indicating that the treated soils were acceptable for "controlled utilization". The mechanism controlling As leaching from all treated samples appeared to be a mixture of wash-off and diffusion. Diffusive As release was proportional to fly ash content. The mechanism controlling Ph leaching when samples were treated with 25% fly ash appeared to be wash-off. (c) 2006 Elsevier B.V. All rights reserved.