Diffusion tests were carried out on compacted soil specimens to investigate the mobility of metals, Cd and Pb, in a tropical residual soil contaminated by acidic solutions at pH 3 and 5.5. After the diffusion, sequential extraction procedure was applied to the soil specimens. This study has shown that the retention of both metals is quite relevant, despite the fact that cadmium is less adsorbed than lead, and that sequential extraction may be a very useful tool to investigate retention processes.