The industrial sites that deal with the production and/or use of cyanide in their processes often have contamination problems by cyanide in soils and water. The behavior of these hazardous compounds in soil and the interactions with soil components are mostly unknown. The toxicity of cyanide and its fate in soil are strongly influenced by the formation of complexes with iron, i.e., ferric - and ferrous cyanides (Fuller, W.H., 1994; Theis and West, 1986). These complexes may interact with the soil inorganic fraction containing ferrous or ferric iron, with consequent formation of stable precipitates (ferric ferrocyanide or ferrous ferricyanide, the Prussian Blue and the Turnbull Blue, respectively). However, no information is available on the capacity of humic substances to immobilize the cy-ano-complexes and thus to reduce their toxicity and the leaching, and the consequent risk of contamination of watertable.In fact, one of the most significant properties of humic substances is their ability to interact with xenobiotics to form complexes of different solubility and chemical and biochemical stability.In this paper we report the results obtained in a voltammetric investigation aimed to study the behavior of the ferricyanide/ ferrocyanide redox couple chosen as the indicator of redox system (Hel burn and MacCarthy, 1994) in the absence and in the presence of humic fraction at nominal molecular weight lower than 5 kDa (HSLMW) extracted from a surface horizon of a Haplumbret soil. The results obtained in these investigations, combined with the evidence gained by spectrophotometric measurements carried out in parallel, point out that ferrocyanide-HSLMW and ferricya-nide-HSLMW complexes are more stable than the parent Fe-CN- ones. These complexes undergo electrochemical processes involving a slow homogeneous chemical reaction preceding the electron transfer step and causing their kinetic control (CE processes). The voltammetric measurements allowed the conditional constants K to be determined for ferric- and ferrous cyanide-HSLMW complexes, respectively: they turn out to be about three orders of magnitude higher than those for the ferricyanide and ferrocyanide complexes alone, respectively. From the data treatment we may hypothize that two humate ligands, on average, are accommodated in the coordination sphere of both iron (III) and iron (II) in the corresponding complexes.