The interaction of several trace elements (Cd, Zn, Cu, Pb, As, Bi, and Tl) was studied by leaching experiments in agricultural soils affected by the Aznalcóllar toxic spill. The spill led to contamination by acid waste waters and sludge deposition. The levels of contamination recorded after the sludge was removed from soils showed that highly contaminated areas remained. A comparison of soils directly affected by sludge deposition and acid waste waters with soils contaminated only by acid waste waters demonstrated that Zn/As and Cd/As ratios were good indicators of the two contributions to the contamination. Soil samples were characterised and grouped according to their texture and carbonate content. The response of elements to single extractions with CaCl₂ 0.01 mol l⁻¹, CaCl₂ 1 mol l⁻¹, CH₃COOH 0.43 mol l⁻¹, and EDTA 0.05 mol l⁻¹ enabled us to estimate their mobility in the soils. Cd and Zn were found to be the most mobile elements. Cu showed an intermediate mobility, especially in an acidic medium. Pb, As, Bi and Tl were found to be non-mobile elements. A comparison of referent, low and highly contaminated samples showed that the presence of sludge had an effect on desorption yields, in part due to the short-term after the contamination. Calculations of a relative scale of long-term mobility, between soils and trace elements, provided further conclusions derived from the use of single extractions.