The speciation of heavy metals (Cd, Cr, Cu, Ni, Pb, Zn, Co, Fe, Mn) in the sludge of an oxidation pond was performed. The effect of sludge maturation on the partition of metals was studied. After ten years of storage, if concentrations of Fe, Mn, Ni, Co are preserved, concentrations of Cd, Cu, Pb, Zn and Cr associated with organic but also easily reducible oxide and oxihydrate phases have decreased.